Cope (1883) -- Exctinct Rodents From N. America

8/12/2019 Cope (1883) -- Exctinct Rodents From N. America

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The Extinct Rodentia of North AmericaAuthor(s): E. D. Cope

Source: The American Naturalist, Vol. 17, No. 1 (Jan., 1883), pp. 43-57Published by: The University of Chicago Pressfor The American Society of NaturalistsStable URL: http://www.jstor.org/stable/2448949.

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1883.] TheExtinctRodeitia ofNorthAmerica. 43THE EXTINCT RODENTIA OF NORTH AMERICA,

BY PROFESSOR E. D. COPE.THE orderofRodentiaappearedin the Wasatch' Eocene epochin NorthAmerica, nd has continued to the presenttime ngradually increasingnumbers. No species of the order is yetknown from the Puerco or lowest Eocene, norare any knownfrom lderformations.The Wasatch -Eocene has given us butfew species, and these are members f a single genus. In theBridger epoch the numberof species was larger, nd theybe-longed to several genera. The order displays a sudden expan-sion in theWhite River or Oligocene epoch. We know fromthisformation enspecies of sevengenera. From the JohnDayRiverformation e have twenty-onepecies of ninegenera. TheUpper Miocene Loup Fork epoch has yielded remainsof ninespecies of sevengenera. Four existing genera are representedby extinct pecies in theMiocene beds; two ofthesebegin intheWhite River and two in theJohnDay epochs.The fourprimary ivisionsof theorder Rodentia are thusde-fined, rincipally fterBrandt and Alston:I. Incisorteeth . Fibula not articulatingwith the superior ondyle f the calca-neum. No intertrochlearrest f humerus.i. Mandible with the angularportion pringing rom heouter ide of thebony coveringof the lower incisor. Fibula distinct fromtibia." Malarbone not supportedbelow bya continuationf themaxillaryzygomatic rocess."' An interpterygoidissure. .HYSTRICOMORPHA.

2. Mandiblewith the angle in theplane of orspringing rom he inferioredge of the coveringof the alveolusofthe inferiorncisor,moreorless rounded; coronoidprocess high,falcate. Fibula distinct romtibia. No interpterygoid fissure.................. SCIUROMORPHA.3. Mandiblewiththeangularportionpringing romhe nferior dge ofthesheathof the nferiorncisor exceptBathyerginoe).Fibula coossifiedwith hetibia. Malar short, sually upported n a maxillary rocess.No interpterygoidissureexcept n Bathyerginae)..... MYOMORPHA.II. IncisorteethA. Fibula articulatingwith hecondyleof thecalcaneum. Anintertrochlearrest fhumerus.4 No true lisphenoid anal; fibula nkylosed o tibiabelow; angle of man-dible in theplane ofthe ncisive lveolus............ LAGOMORPHA.These groups,as is well known, nclude familiesand generawhich display adaptationsto various modes of life. Some areexclusivelysubterranean,thersare arboreal, nd some live onthe surface ftheground. Of the latter, ome are providedwithformidable pines as a protectionagainst enemies,while othersdependfor heir afety ntheir peed. Of the latter haracter re

I For the positions of the American Tertiary epochs, see AMERICAN NA-rURALISTSi882, March.

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44 The &1tincLRodleai'ez fAoart/tinerica. [January,the Rabbitsof the Lagomorpha,and I have noted'how thattheyhave superadded to the ordinary odent structure ertainpointswhichalso characterize he most specializedPerissodactyla ndArtiodactyla mong ungulates.The fusion f the inferiorartofthefibulawith hetibia found lso in the Myomorpha)belongstothehighertypes ofthese orders. The strongntertrochlearidgeofthe humerus s an especialfeature f thegroupsmentioned, is-tinguishing hem from the lower types n all the orders. Thearticulation f thefibulawiththe calcaneum, mentionedby Mr.Alston, s a character ftheArtiodactyla.Associatedwith heseistheelongationof thebones of the limbs,especiallythe posteriorone. The modification f the tarsus in Dipus (the erboas) evi-dentlyhas a direct relationto the projectileforce transmittedthrough hehindlegs in rapid progression y leaping. Here themetatarsals re co6ssifiednto cannonbone,though, s there rethreebones involved, he result is somewhat differentrom hecannonbone of the Ruminantia.

Wastc Brdgr.White John LoupWasatch Bridger- River. Day. Fork.HYSTRICOMORPHA porcupines).Hystr;icidev.Hystrix L...... ...... .....SCIUROMORPHA squirrels).AMyl/agauilidw.MylagaulusCope ...... .... ..... ...... ..... ...... 2]zzm. ?HeliscomysCope..... ....... ......Casio; de. .EucastorLeidy............... ... .......... ...... ...CastorL............ ...... ......fIsc~zyromooyid- .Plesiarctomysrav. ..................... 3 7 ......SyllophodusCop( ....................... 2 ......Ischyromyseidy. ........ ........ ..I............ .... ...... ......Sciurid.Meniscomys ope . ............ 1 4Gymnoptychusope . . ...... ... 2 . ......Sciurus Linn ........................... ...... ...... I 2 I

MYOMORPHA (rats).Murde.l lEumysLeidy ................ ...HesperomysWaterh............. . ... ..... I IPaciculusCope ........ . ...... 2Geomyidt-e.PleurolicusCope ....... 3 ......Entoptychus ope ....................... 5LAGOMORPHA rabbits).Lepori(hv. iPaleolagus Leidy .....L....... 4Panolax Cope .................... ...... ILepus Linn ... ....... ........ ..... II BulletinU. S. Geological SurveyTerrs. v, 362, i88i.



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1883.] TheExtinct odentiafNorth merica. 45After generalviewof the speciesand genera,ome deduc-tions s tothecourse f evolution f theorderwillbe presented.

EOcENERODENTIA.PLESIARCTOMYSravard.This is theprevalent enusof Rodentia f the Eocene periodin NorthAmerica. Specimenswere firstdiscoveredby Dr.Hayden in Bridger edsof Wyoming, nd were described yDr. Leidy. I subsequentlyetected hemntheWasatch orma-tionof New Mexico. Theirremainsre rather bundantn bothformations,utdisplay ut ittle ariety f form.Theteeth ave hort rowns nd long roots, ndhavethe gen-eralcharacterss wellas numberss the xistingpecies f quir-rels. There re,however,ranial haracters hich istinguishtfrom heexisting orms f thatfamily.The crowns f the nfe-

FIG. I.-Parts of two crania and the ulna and radius of Plesiardomys elicatissi-musLeidy,natural ize, from blockof the Wasatch bed of the Big Horn river,Wyoming. Original, romVol Iv, ReportU. S. (eol. Surv.Territories.riormolars upport our ather mall nd strictly arginaluber-cles. There refiveuperiormolars, fwhich heanteriors ofsmall ize. Theyresemble hoseof Sciurus, ut the transverse



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46 TheExtinctRodentiaofNorthAmerica. [Januarytcrestsare obsolete or wanting. The positionscorresponding otheirexternal extremities re markedby more or less distinctcusps. There is a single internaltubercleof the crown. In thethird nd fourthmolarof P. delicatissimus observerudiments fa second internal ubercle. The incisor teeth are compressed,with narrow nteriorface. The enamel is not grooved,and islittle or not at all inflected n the inner ide of the shaft,while itis entirely o on theexternalface.There is a large roundforameninfraorbitalexterius,ike thatof Ischyromys nd Fiber, and entirely nlikethat of Gymnopty-chus and Sciurus, conformingn this respectto the forms f theextinctgroup of the Protomyidae f Pomel.The cast of the brain ndicates moothoval hemispheres,whichleave the cerebellum and olfactoryobes entirely xposed. Thelatter re ovoid and expand-ed laterally.-- O1 & ^ The species from whichmost of the characters f thegenus as above stated havebeen derived are the P. deli-4-- 3 \^,catior and P. delicatissimus.l I At Theyfurther isplaythe fol-it I\ ~lowing general characters.The anterior imbs are-rela-

tively longer than in recentspecies of squirrels. Thehead ofthe radius s rounder,indicating n unusual powerof rotation of the anteriorvAI&>0.. limb. The pelvis is larger,_ b J~~~~~ beingas long as the skull,and it is probable that the/posters ilimbcis larger.These/ points ndicateapproximationFIG. 2. to the cotemporaryMeso-donta,or half emurs.FIG. 2.-Bones of the specimensof No characters have yetardiemysdelicatissimus eidy, representednFig. i. Fig. a, humerus, ront iew; b, prox-been offeredy whichto dis-imalpart of ulna and radius. Fig. c, distalpartof tibia posterioride; d, same frombe- tinguishthe American spe-

low; e, astragalusfromabove; f, astragaluscies as representing genusand calcaneum,istalends. Original romVol. Iv, ReportU. S. Geol. SurveyTerrs. distinct from the Plesiarcto-



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I883.] f/i Evrtiic Rodentia f z.ot/i Amcri'ca. 47mys gei'vaisii of *the French Eocene. Bravard brieflydis-tinguishesthe genus as distinctfromArctomysin the greaterthickness f the angles of the molars,which thusbecome tuber-cles. Only the mandible and mandibular teeth of the P. ger-vaisii are known. It has been found in the Upper Eocene, nearPerreal,Apt, France.I have seen six species of this genus,of which two,P. hiansCope, and P. undans Marsh, belong to the Bridgerbeds only;one P. lep/odusCope, to theWashakie; one P. buccatusCope,to the Wasatch and Wind river, nd two, P. delicatiorLeidy, andP. delicatissimus,eidy, to all theEocenes except theWashakie.The following omparison fthe P. delica/issimusith heSciurusniger, r commongray quirrel,maybe made. The pelvis s longeras comparedwith hebones of he fore eg. The humerus s longeras comparedwith he lengthof the ulna and radius. The speciesexceeds the S. niger in size, one-fourthinear.It is then probablethat the species of this, the oldest knowngenus ofRodentia,were arboreal, ike the squirrels'of hepresentgeological period.SYLLOPHODUS Cope.This genus is much like Thzeridomylsf the European UpperEocene and Lower Miocene,and maybe the same. The specieswere smaller hanthose of the last described, nd are only knownfrom ower jaws. These contain teethwhich differ romthoseof Plesiarctomys n having cross-crestswhich are slightly on-nected at one side. They look like the unwornconditionofIschyromys, f which genus they may be the ancestor. Twospecies,S. minimiusndS. fraterizus ave been describedby Leidy.Both are from heBridgerhorizon.

MIOCENERODENTIA.ISCHYROMX'Seidy,The essential eaturesre,dentition,. 1; C., ?; M., 4; the molarswithtwocrescents n the inner side above, each of whichgivesriseto a cross-ridgeto the outer margin. In the mandibularseries the crests and crescenthave a reversedrelation. Nocemnentum.To the above charactersgiven by Dr. Leidy, I have added theabsence ofpostfrontalrocesses,and the superiorpositionof theinfraorbitaloramen. Also thatthepterygoid ossa is large,and



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48 TheExtinct odentiafNorthAmerica. [January,that ts innerand outerplates are well developed,nd sub-equal.The bones of the limbsare generally imilar o thoseoftheSciuridc. In thisfamilyhegenusGymnoptychusearlyesem-blesIschyromysndental haracters. But besides he mportantdifferencentheformernd position f the infra-orbitaloramen,Ischyromysas an excavated osterior alatalborder.

FIG. 3..-Ischyromys ypus eidy,natural ize,fromheWhite riverbedsof Color-ado, original, rom heReportU. S. Geol. Surv.Terrs. , b, c, cranium; d, mandiblefrom bove.Dr. Leidyremarkshat hisgenusbelongs othefamilyftheSciuride. This s indicatedythedental haracters.but n someother espectsheres a greater ivergenceromhe quirrelsndmarmotshan s thecase withthegenusGymnoptychus. hus,the large foramen nfraorbitalenterius ccupies the elevatedpositiont theorigin fthezygomaticrch een nthe orcupinesandcavies. There s no superciliaryidgenorpost-orbitalro-cessas in most ciuride,butthe fronts contractedetweenheorbitsn thesame manners,but to a lessdegreethan,nFiber,andtheEocene Plesiarctomysrav. Both the last namedandIschyromysresentmany oints fresemblanceo Pomel'stribe



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1883.] TheExtinctRodlentiafLNors America. 49of Protomyide,but differ rom ny of the genera he has includedin it.This familys thus defined y Pomel 1 "Infraorbitalforamenas largeas in theHystricidce,nd by the position of the angularapophysis of the mandible almost n the generalplaneof the hori-zontalramus. The jugal bone,at least in thosespecieswhere wehave observed t, s very much enlarged at its anteriorportion,and theorbit s almost superior."These characters pply to Ischyromys,excepting as regardsmalarbone,whichis principally nknownn the latter.Another family,the Ischyromyidce, as been proposed byE. R. Alston,for hereception f thisgenus, to which he thinkswith me2 Plesiarctomys =-Pseudotomus) should be referred.He thus defines hefamily3 " Dentition s in Sciuridae; skull re-sembling Castoridx, butwith the infra-orbital pening large, asagittalcrest;no post-orbital rocesses; palate broad; basioccipi-tal keeled."Doubtless Ischyromys elongs to an extinctfamily, utwhichoftheabove names is availablefor t I do notyetknow. I wouldcharacterizet as follows:Dentition s in Sciuridx, infraorbitaloramen arge, superior;pterygoidfossa large,withwell-developedexterior s well as in-feriorwalls; a sagittalcrest.The superiorpositionof the infraorbitalorarnennd thewell-developed pterygoid laminae are characteristicsfound in theMuridae.But one speciesof this genus is known,the Isc/iyrom syplsLeidy. The skull is as largeas that of a prairiemarmot. Thelimbs re comparatively mall, o thattheanimal was notprobablyarboreal n itshabits.SCIURusLinn. (true squirrels).

In this genus the molarsare 4 or 4, the firstsuperior smallwhenpresent. The grinding urfaces f the crownswhenunwornpresentn thesuperior eriesa single internal usp,whichis lowand anteroposterior. From this there extend to the externalborderof the crowntwo low transverseridges,whose exterior1CatalogueMethod.et Descr. de Vertebres oss. (le le Bass. de la Loire, 1853,p.

32.2AnnualReportU. S. Geol. SurveyTerrs.,1873 (1874), p. 477.3 Proceed.Zool. SocietyLondon, 1876,p. 78.

VOL. XVII.-NO. 1. 4



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50 TheExtinct odentiafNorth merica. [January,terminationsresomewhatnlarged. In the ower awthe rans-verseridges re notvisible,nd there s a low tuberculeteachangle of thecrown, etweenwhichtheremaybe others n theborder fthecrown. Attritionives hegrindingurfacef thelatter basin-likeharacter.Theforamennfraorbitales a short,narrow issure,ituatedn the nferior art f themaxillary onein frontf its tooth-bearingortion,ut descendingo the evelof he lveolar order.Thewell-knownharactersfthisgenus refound n theman-

dibles f specieswhich ob-t~ q tainedfrom he White River_' IMiocene beds of Colorado_ " and theJohn ay ofOregon.a~ r bd The teeth display he sub-quadrateformf this genus,without nytendencyo thetransversenlargementeenin Arctomys, ynomys,ndSpermophilus.Two of the_ e _-a species,.vortmaniope ndS. relictusopeare s large sour grayand red squirrels,

FIG. 4.-Skull and awsofspecies fSci respectively,nd the third, .urus; original,romol. vReport . S. Ge. ballovianusope,s about heolog. Surv. Terrs. Figs. a-d, S. ballovianus; size of the Tamias quadrivi-bandd, superiornd nferior olareethn-larged. Figs.e-f, . relicdus,nlargedne-tatus or Western hip-munk.half. g-h, S. vortmani,aturalize. The S. relictus s from heWhiteRiverformation,ndthetwoother peciesfrom heJohnDay.GYMNOPTYCHUS Cope.In dentition hisgenus smuch ike Ischyromys.Thereareonlyfouruperiormolars.As compared ith heexisting enera fsquirrels,t differsnthe tructuref the molar eeth. The arrangementf thetuber-cles and crestss morecomplex han n anyof them, xceptingPteromys.Thus inall ofthem heres butone internalrescentofthe uperiormolars,ndbuttwo r three ross-crests; hile nthe nferior olars hearrangements unlike hatof thesuperiorteeth, he cross-crestseingmarginal nly. In PteromysF.Cuv.)thetransversealleys fthe inferioreries ofGymnopty-



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I883.] The ExtinctRodentiaof NorthAmerica. 5Ichus are representedby numerous iso-lated fossettes. The structureof themolars in the fossil genus is exactlylike that which I have described asfoundin Eumys, extendingeven to thedetails. This is curious, s that genusis probably Myomorph.The protrusionof the posterior ex-tremity f the alveolor sheath of the in-ferior ncisor on the outer side of theascendingramus s not exhibitedby theNorthAmerican Sciurida which I haveexamined,norby any of the extinctgen-era herein described, excepting Castorand the Geomyida. It is seenina lesserdegree n the house and wood mice, thejumpingmouse nd meadowmouse, ll FIG. 5.--d, GymnoptychusMuridae. minutesope, rom heWhiteRiver bed of Colorado. a,Two species fthis enus recertainlyatural ize; b-d, enlarged,known. They belong to the White , ower awof Gymnoptychustritophusope, from bove,River horizon f Colorado. They dif-nlarged;ame ocality.Orig-fer, o far as known, hieflyn size,and final.in the proportionsof the inferior remolar tooth. See Fig. 5.MENISCOMYSCope.This genus is readily distinguishedfrom all the others heretreated f,by the complexity f the structure f its molar teeth,and the curious resemblances that some of them present o themolarsof the hoofedmammalia. They are without namelinflec-tions, nd the trituratingurfaceexhibits two external nd oneinternal rescentic ections of the investing namel. On the sec-ond superiormolar thereare three external crescents, nd thefirstmolaris simply onic. Between the inner nd external res-centsthere are the curved edges of enamel plates directedob-liquely and transversely. The grindingsurfaces fthe inferiormolarsdisplay, n the unworn ondition, urved transverserests,connected longitudinallyon the median line; on wearing,thelateralemarginationsof the enamel become shallower, disap-pearingfrom heinner ide,but remaining n the outer. Incisorteethnotgrooved. Foramen infraorbitalenterius mall inferior,



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52 TheExtinctRodejtiacf.NortliAmerica. [January,and near the orbit. Postorbitalprocesses; no sagittal crest.

-e The characters fthedentition f thisgenus resembles those of the genus_ Pteromys,which includes the large fly-ing squirrels f Asia and theMalaysianarchipelago, to which region they areconfined. The superior molars differ1b -from those of Pteromys n wantingallre-entrantnamel inflection.- The generalcharacters f theskeletonc are unknown. A femur s ratherslen-der, and a tibiarather longate, howingthatthe limbs are notshort.Four species of thisgenusare knownto me, all from he JohnDay Mioceneof Oregon. They differ onsiderablynthe details of the structure f the mo-e lar teeth. Those of the Meniscomys

FiG. 6.-Craniumi, aws and /ippodzusre more prismatic hanthoseteethof Meniscomys Aipodns f the other species, and the externalCope,from he JohnDay bed . - . *aof Oregon; natural size and face s not inflected t thegrinding ur-enlarged. face as in them.Neverthelesshe molarshave shortroots. The arrangementf thecrestsof the crown ofthe superiormolars is a good deals a like that to be seen in the molars ofsome of the later three-toedhorses,if the cementum e removed. (Fig.6.)In the A. catvatus Cope (Fig. 7),theconstitutionf he uperiormolarsbrn is morecomplex, hilethat of theinferiormolars s more simple. Thebulla of the ear is set with simpleC transverse epta within,while in theM. Izippodus heir internalface hasa reticulate tructureiketripe. Thed_ < superior molars of the AL ZiolophusCope (Fig. 8, a, b) have theircrestsFIG. 7.- Offeniseonly~savaliesCope, part of cranium nd lower and cusps unwrinkled. In the M.

jaw ofone ndividual romheJohnni/ens Marsh,theyare complex andDay river,Oregon; nat. size andenlarged. Original. muchwrinkled, hile he owermolars



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1883.] TheExtinct odeutiafNorthAmerica. 53are most complex with reticulateridges. (Fig. 8,c.)

There s a suggestive esemblancebetweenheformsf hemolar eethoftheMeniscomysikpodusndthoseof the Haplodontia ufa now ivingin Oregon. The two genera havedoubtlesshad a common rigin, utthepresent ifferencesreconsider- bable. ThustheHaplodontiahas anextendedosseous cavum tympani,whichdoes not exist in Menisco-mys.CASTOR Linn. FIG. 8.-a-A, superiormolarsofThe beaver s the largestrodent enzscomysiolophus, nlarged. c;ofthenorthernemisphere,ndhas inferior olars f M. ni/ensfromabove,enlarged. d, skulloftheMl.thewidestdistribution.t was pre- zppodus,,nat.ize. Original; fromtheJohnDay river,Oregon,ceded in the Miocene period by a t

0

FIG. 9.-Skull of Castor eninsulatus Cope, nat.size From the JohnDay epoch,Oregon. Original.This content downloaded from 201.151.199.254 on Thu, 22 Aug 2013 17:17:03 PM

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54 TheExtinct odentiafNorthAmerica. [January,numberof species in both the eastern and westerncontinents,of inferior ize and more restricteddistribution. The greaternumber of these belong, I believe, to the same genus asthe Castor fiber, thoughthey have been separatedunderthename of Steneo-

- ~~fiber eoffr. There is noessential differencen thedentition,nd it is probablethattheextinct pecieshadthepeculiarflat tail of themodernbeaver. The cau-dal vertebrae f the C.pan-h sus, rom Nebraska, haveexactly the characterofthose of the beaver.

The familyf the Castor-idcediffers rom heSciuri-da in the absence of post-orbital angles or processes,and the presence of a pro-longed tube of the meatusauditorium externus. Inbothof hesepoints tagreeswiththe Haplodontiidae,family hichMr.Alston hasdistinguished rom heCas-LIX gtoridaw on variousgrounds.I do not thinkany of hischaractersare tenable, x-cepting that drawn fromthe formof the mandible,which s expressed thus inMr. Alston's diagnosis:FIG. io.-Skull and bones fCastorpenit "angularportionfmandi-sulatus ope,representedn Fig. 9. a, occi- . ,,pital iew; 6,c, right amus fmandible;d, ble much wisted." Thisrightemur.Naturalize. character illbebetter e-

scribed as follows: Angle of mandiblewitha transverse dgedue to inflection n the one hand, and productionnto an apexexternally;the nflectionoundinga large nterno-posteriorossa.



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l 883.] The ExtinctRodentiaofNorthAmerica. 55The Haplodonta rufa is a curious burrowingrodentfoundinthe Cascade mountainsof Oregon, and is known as showtl orsewellel. It has no tail.The longest knownspecies of this genus is the C. viciacensis,which is common in the Miocenes in several parts of Europe.In North America the C. nebrascensis eidy, is statedby Hay-den and Leidy to be found n the White River formation. It isofabout the samedimensions s theEuropean species. So are theC. peninsulatus ope, from he JohnDay Riverepoch ofOregon,

and the C.pansus oftheLoup Fork horizon f aNew Mexicoand Nebraska see Fig. I I). Thesmallestspecies is the C. gradatus Cope, acontemporaryf the C. peninsulatus n Ore-gon. None ofthesespeciesarenearly o largeas the recent beaver.EuCASTOR llen.

Besides thepreceding,here re some otherforms f beavers n the ateTertiaries fNorthAmericaand Europe. bThe Castor tortuswas describedby Leidyfrom he Loup Fork formation f Nebraska. bHe coined thesubgenericname Eucastor forit withoutcorrespondingdefinition. In hismonographof the Castoridae, . A. Allen re-ferred his species' to a genus distinctfrom cCastor, nd definedt, using for t Leidy's Fig. I.-Castor pan-nameEucastor. This genus appearsto me to su Cope, Loup Forkname ~~~~~~~~~~~epo-ch;, b, fromNewbevalid. The three enera fCastoridewillMexico;c,caudalverte-brafrom ebraska. atthen be defined s follows: size. Original.Molars and premolarswithone innerand twoor three uterfolds........ Castor."Inferior premolar nd third uperiormolarelongate,with four namel folds; therestwithonlytwo ................ - Diobroticus.Superior remolar nlarged,withone inner fold; inferiormolarssmall,with two

lakes ... ... Eucoastor.

1Monographs f NorthAmericanRodentia,Coues and Allen, U. S. Geol. Surv.Terrs., 877, Xi, p. 450.



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56 TheExtinctRodentiaofNorthi merica. [January,The Eucastortortuswas largerthanany of the extinctspeciesof Castor,but was considerably maller than the beaver. The

Diobroticusrogontheriumf Europewas a still arger pecies,one-fifthargerthan thebeaver in dimensions.MYLAGAULUS Cope.The reductionof the posteriormolars, seen in Eucastor, iscarried to a still higher degree inthisgenus. The last or fourthmo-larhas disappeared, nd the indica-

tions fromthe specimensare, thatthe third was earlyshed. The sec-*.% _ _ _ ond is a small tooth,while the firstis enormous, and performedthegreaterpart of the functionf mas-tication.* The charactersof the genus are:I d Inferior olarshree,ootless; hefirstmuch larger than the others.Enamel inclosing hefirstmolar notFIG. I 2 -a-, b,c, M~ylavgEstZJIG. 2.-~ b, ,laaulus 7mo01. inflected; butnumerousfossettes nodon Cope, lower aw and a separatetooth,natural size; d, M. sesquipe. thegrindingsurfaceof the crown,dalis,firstnferior olar, at.size.Original. From heLoup Forkepoch whose longdiameter s anteroposte-of Nebraska. rior.The onlylower aw ofa species of thisgenus inmypossessionpresents small partof thebase oftheangle and of the coronoidprocess. These parts are so nearly n the plane of the incisivealveolus as to lead to the belief that the genus Mylagaulus be-longs to the sub-orderSciuromorpha. The rootlessteethwithdeep enamelfossettes pproximatest to the Castoridae, ut itap-pears to me thata new familygroup must be establishedfor tsreception. Such charactersare the presence of only three in-

feriormolars, nd the entire ndependence f the enamelfossettesof the external sheathing namel. It is worthyf investigationwhether he flystrix efossaGerv. has any relationto thisfamily.Two species of thisgenus are known: a larger M. monodonCope, and a smaller,M. sesquipedalisCope. Both are from heLoup Fork epoch of Nebraska. The formerwas about the sizeofthewood-chuck Arctomysmonax), o udge by thedimensionsof ts lower aw. It is largerthantheM. sesquipedalis,nd has adifferentrrangementf the enamelfossettes.In thatspecies, in-



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1883.] Yke ExtiuctRodentziz fNow/ll zliesi'cz. 57stead of beingin threeparallellines,themiddle ine s only repre-sented by its extremities. At the middleof the crown the fos-settes of the internaline are incurved o as to be nearly n con-tact withthe fossettes f the external ine.HELISCOMYSCope.Inferiormolars four-rooted,he crowns supporting our uspsin transverse airs. A broad ledge or cingulum,rojecting n theexternal ide from ase of the cusps. The inferior ncisor com-pressed,notgrooved, nd with the enamel,without culpture.

FIG. 13.-a(al, lower aws of Ike/iscomys et/us ope; a, natural ize; b,c, d, en-larged e,f, Eitmnys legainseidy,natural ize; e, cranium rombove;f, left amusoflower aw, external ide. All from heWhiteRiverepochofColorado. Original.This genus is onlyrepresentedby a small numberof speci-mens,which are mandibular amiexclusively. Its special affini-ties thereforeannot be ascertained, nd even its generalpositionremains omewhatdoubtful.There is some probability, owever,that tbelongsto theMyomorpha, s thetypeofdentitions muchmore ikethatofthegeneraof thatgroupthan those of the Sci-uromorpha. To theHystricomorphat does notbelong.As comparedwithknowngeneraof Myormorpha,t is at onceseparatedfrommanyof thembythepresenceofa premnolarooth.Among recentgeneraof thissub-order, minthuspossesses thistooth n both jaws, and Meriones in the upper aw only. It ispresent n both aws in the Sciuromorphagenerally. The tuber-cles of the teethresemble hose of the Muride, but theirdisposi-tionis unlikethatofanyexistingNorth Americangenus. A re-moteapproximationo it is seen in thegenus Syllophodusof theBridger Eocene formation,where there are four subquadratemolarswithtubercles; but the latter form wotransverse rests,withan additional mall intermediate ubercle, nd the wide cin-gulum. s absent.But one species ofHeliscomys sknown, heH. vets, from heWhiteRiverepochofColorado. It is not arger han hedomestic

mouse Mus mutscdus).(To becogfinittic.)

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Cope (1883) -- Exctinct Rodents From N. America