284

AN ABNORMAL CHALK FORAMIMFERBy TOM BARNARD, B.Sc., Ph.D., D.l.C., F.G.S.

[Received 10th June, 1949]

SUMMARY-Unlike other free-living foraminifera some specimens ofFlabellina cf angulosa d'Orbigny from various zones of the Upper Chalk show astrong tendency to have adherent end chambers. This together with a changeof ornament from smooth to a pustulose shell seems to be unique amongstforaminifera.

INTRODUCTIONDURING a preliminary study of foraminifera from the Chalk,

contained in the A. W. Rowe Collections at the British Museum(Natural History), several abnormal specimens were found. Someof these showed interesting features, which are considered worthyof note. Many foraminifera are known to have irregular abnormalend-chambers, or to grow quite regularly until a certain stage isreached, and then suddenly to become irregular and begin a newpattern of growth.

Some species of Ammodiscus from the Lias and Oxford Claycontinue planospirally for several whorls, until the end stage isreached, when the continuous tube suddenly turns back and con­tinues in the opposite sense. This type of irregularity has also beennoticed in some specimens of Comuspira,

Amongst the polythalamous foraminifera many examples ofirregularity may be cited. Specimens of ornamented Nodosariahortensis Terquem, from the davoei zone of the Lower Lias, oftenhave a regular growth broken by the appearance of a much smallerchamber, which seems to occur at any growth stage, after which thetest returns to its original growth rate. In this case there is noapparent injury to the test.

Reversal in the sense of direction or coiling of the test has alsobeen observed in some species of Ophthalmidium.

During a detailed study of the family Lagenidae the authorhas noticed a prevalent tendency for the end-chambers in manyspecimens to become abnormally small or abnormally large, butto keep the shape of the earlier chambers. The end-chambers ofother specimens show an extreme irregularity in which the shape ofthe last bears little or no relation to those occurring earlier. Thesefeatures are well seen in specimens of Dentalina, Lenticulina, Planu­laria, Flabellina and Frondicularia. Occasionally specimens ofLenticulina have their end-chambers arranged so that the axis ofcoiling of the spire is at right-angles to the earlier part of the test.

All the genera just mentioned are benthonic and free-living.As far as I am able to ascertain the change from a free-living to anadherent mode of life in the same individual has not so far beendescribed.

AN ABNORMAL CHALK FORAMINIFER 285

Certain specimens of Flabellina have been found attached onlyby irregularly developed end-chambers and it is here proposed todescribe some of these forms.

SYSTEMATIC DESCRIPTIONSFlabellina cf. angulosa d'Orbigny

Description. Several specimens were selected from an abun­dance of this species, to illustrate the intergradation from the freeto the adherent form.

(a) Normal Specimen. Plate 12, Fig. 1. (PA0270).IThe initial megalosphere is a spherical chamber which stands

out above the general flattened surface of the test.Three chambers follow in a very loose spire, the sutures are

rectangular, and raised above the chamber wall. The" apertures ..of these chambers are marginal. The chambers are arranged sothat they are distinctly evolute on the apertural margin, but tapertowards and overlap onto the proloculum. This is the "cris­tellarian" growth-stage.

In subsequent regular chambers the aperture moves from themarginal to the central terminal position, and this is accompaniedby a rapid change to the chevron-shaped chambers typical of thegenus. The sutures remain visible but become less and less raisedabove the surface of the test. The edges of the test are arcuate andnot straight as in many species of Flabellina.

The final slit-like aperture is on a thick cylindrical neck.Horizon: Belemnitella mucronata Zone.Locality: Tharston, Norfolk.

(b) Specimen showing Abnormal Flabelline growth-stages. Plate 12,Fig. 2. (PA027 1).

The megalospheric specimen has the three initial chamberssimilar to the above. The specimen then shows a complete reversalin the direction of coiling. The fourth chamber overlaps ontothe apertural margin of the earlier chambers, the aperture changesfrom this apertural margin to the opposite side which also becomesevolute.

Two regular smooth, curved, chevron-shaped chambers follow.A change is also noticeable in the development of the sutures.

In the earlier chambers the sutures are raised, later becoming levelwith the surface of the test and finally depressed below the surface.

Horizon: Belemnitella mucronata Zone.Locality: Tharston, Norfolk.

I Registered numbers referring to specimens preserved' in the collections of the GeologicalDepartment of the British Museum (Natural History) are shown thus: P.-.

286 TOM BARNARD

Fig. 5Belemnitella

mucronateZoneTharston, Norfolk

Fig. 4?Belemnitella

mucronata ZoneStonehill Kiln

NorwichLocality:

Horizon:

(c) Specimens with a "Ramuline" Trend. Plate 12, Figs. 3, 4, 5.(P.40272-4).

These megalospheric specimens commence growth as in thenormal form, but after a few chambers a completely differentchamber with a " Ramuline" growth follows.

In the specimen (Fig. 3) the first and subsequent chevron cham­bers become swollen into globose chambers, at the same time thereis a change from the unornamented initial stage to a hispid ornament.

In the specimen (Fig. 4) several hispid chambers, of unequalsize, are developed. Another irregularity occurs here. After theinitial regular stage, there follow two hispid chevron-shaped cham­bers arranged so that they are in a plane at an acute angle to theoriginal planispiral development. The three last-formed chambersare arranged in a different plane almost at right angles to those thatprecede.

There are then three distinct planes of growth.The specimen (Fig. 5) shows a development of the regular

normal test, and three regular chevron-shaped chambers occurbefore an abnormality is found. Then the smooth unornamentedchambers are followed by chambers ornamented with very strongpustules, much stronger than in the hispid forms.

The first chevron-shaped chamber follows in the general directionof earlier chevrons, but this is followed by two chambers arrangedin the same plane as those preceding, but at right angles to, thegeneral vertical axis of growth.

Fig. 3?Belemnitella

mucronataZoneCouncil's Pit

Newmarket RoadNorwich

(d) Adherent Specimen. Plate 12, Fig. 6. (P.40275).This megalospheric specimen commences with a swollen globose

proloculum, which is followed by only one" cristellarian " chamber.Three normal chevron-shaped unornamented chambers follow, withraised sutures. This" initial" stage appears to be raised slightlyabove the surface of the echinoid to which the test subsequentlyadheres. At this stage no attachment to the echinoid can be seen,and it is highly probable that the test was free, as in the specimensdescribed above. This portion of the test is then followed by ninechambers. At first these later chambers grow at right angles to theearlier direction of growth, but later the direction is meandrine.

All the chambers (except those badly preserved) show the" ramuline" pustulate ornament. Although still retaining achevron-shape, the chambers become greatly swollen, so that incross-section. they appear almost hemispherical.

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Growth stages of Flabellina cf angulosa d'Orbigny,

AN ABNORMAL CHALK FORAMINIFER 287

The chambers are now distinctly adherent, and each possessessmall calcareous protuberances which connect with the smallertubercles and granules of the echinoid test.

Several specimens were found showing this character, so thatthe adherent trend in the" old-age" stage is by no means rare.

Horizon: Belemnitella mucronata Zone.Locality: Tharston, Norfolk.

The writer wishes to thank Mr. C. D. Ovey for reading thetypescript.

EXPLANATION OF PLATE 12Specimens of Flabellina cf angulosa d'Orbigny showing different growth­

stages. All specimens magnified x 18.Fig. 1. This specimen shows the initial coiled stage, followed by a vaginuline

growth-stage, and finally one flabeIline chamber.Fig. 2. The coiled and vaginuline stages are omitted, and the normal flabeIIine

growth makes up most of the test.Fig. 3. A young specimen in which the flabelline is replaced by a hispid

ramuline growth-stage.Fig. 4. Similar to the above but several hispid flabeIline chambers make up

most of test.Fig. 5. A large specimen of normal form, coiled, vaginuline and flabelline

growth-stages, followed by one swollen hispid chamber.Fig. 6. This specimen has an initial shell composed of the three normal

growth-stages followed by a number of adherent hispid chambers,the earlier ofwhich stilI possess an inverted V-shape, in later chambers,however, this is lost. Small protuberances adhere to the granulesand tubercles of the echinoid to which the foraminifer adheres.