of ribs directed away from a dividing line, so clear a feature in the figure, is quite erroneous. In the specimen itself, an appearance of dichotomy occurs only in one rib, and this seems to be due to an accident of preservation. There is a crack in the shell, and a portion of the valve on one side has become pushed slightly over the surface on the other side of the crack. From an examination of the specimen it is difficult to say with certainty whether this injury took place during the life of the animal, or subsequently, but the resulting irregularity in the sculpture was in any case not a normal character.

There is a close general resemblance between L. (Mantellum) neglecta and L. parallela J. de C. Sow.; but in Lima neglecta the oblique elongation is greater and the posterior ribs are more crowded and delicate. Lima (Mantellum) gaultina Woods is more closely similar to L. neglecta in outline, but its antero-ventral border is more narrowed, and the fine linear rib in the interspace between the main ribs is lacking.

GENUS PERNA J. G. Bruguière.

PERNA ATHERSTONI Sharpe.
Plate II., fig. 9.

1856. Perna atherstoni D. Sharpe, Trans. Geol. Soc. Lond., ser. 2, vol. vii., p. 193, pl. xxii., figs. 4, 4a.

Occurrence. This shell was found at Dunbrodie (Sunday's River), in the cliff below the old school-house (282, 305, 320), and at Walton's Farm, below Dunbrodie (311). It was recorded by Sharpe from the same place (Geelhoutboom) "in greenish grit with fragmentary shells."

Remarks. These specimens from Dunbrodie were thought to represent very probably a new form, but a comparison with Sharpe's original specimens of P. atherstoni shows that they differ from these in no essential particular. One of the Dunbrodie individuals, it is true, shows a slight difference in outline; its antero-inferior border, at a distance of 15 mm. from the umbonal apex, has a broad indentation or inward curve in profile, somewhat more strongly developed than in the original of Sharpe's figure 4. Other specimens which I have examined exhibit this feature in less degree, and it appears to be a character which is subject to some individual

* For full description and references to literature see Woods (3), vol. ii., part 1, p. 28, pl. v., figs. 14, 15 (1904).

† Woods (3), vol. ii., part 1, p. 31, pl. v., figs. 16–20 (1904).

variation. The convex fold of the valve, passing back from the umbo, is marked off from a small, flattened, marginal portion of the valve between the fold and the antero-inferior margin near the umbonal end, in clearer manner as a rule than is depicted in Sharpe's figure. In this respect the figure is misleading, for the fold is well developed in the original specimen. In another point, also, the illustration leaves something to be desired; the posteroinferior border, restored by a dotted line, should in reality have been represented as a rounded curve similar to that followed by the outline of the same part in Perna mytiloides Lam., and should not have been drawn in the angular or sub-angular form which lends such a false aspect to the outline of the shell as depicted in Sharpe's work.

P. atherstoni is characterised by its oblique figure and the relatively great length of the hinge-line. In these features it recalls P. mytiloides Lam., from the Upper Oolites of England, which in general aspect it much resembles; but in P. atherstoni the shell is less sharply pointed and produced at the umbo, is less inflated anteriorly, and is more obliquely elongated. There are further points of difference which it is unnecessary to recount.

GENUS PINNA Linnæus.

PINNA ATHERSTONI Sharpe.

1856. Pinna atherstoni D. Sharpe, Trans. Geol. Soc. Lond., ser. 2, vol. vii., p. 193, pl. xxii., fig. 1..

Occurrence.-Railway cutting between milestones 241-24 on the railway from Uitenhage to Graaff-Reinet, about three miles from Uitenhage (329, 330), where it is very abundant in a bed of nodular limestone.

Remarks.—It has been noted by Messrs. Rogers and Schwarz that this shell, associated with Holcostephanus atherstoni, marks the most constantly recognisable zone in the Marine Beds of the Zwartkop's River valley.

Pinna atherstoni was compared by Sharpe and Tate with P. hartmanni Zieten, from the Lias of Europe, and it certainly agrees more closely with P. hartmanni, as figured by Goldfuss,† than with the generality of Cretaceous forms, most of which are more elongated and slender in outline. The original figure given by Zieten

* Damon (1), pl. ii., fig. 5.

↑ Goldfuss (1), Band ii., p. 164, Tab. cxxvii., fig. 3b (1837).
Zieten (1), p. 73, Tab. 55, fig. 5 (1833).

has, however, a very different aspect from that of Goldfuss; it apparently has a narrower apical angle, and the surface ornaments are so delineated as to be scarcely comparable with those of P. atherstoni. The resemblance of P. atherstoni to the Liassic form usually known by Zieten's name is clearly a fact of no importance in the question of the age of the Uitenhage fauna. It may be noted that a specimen of Pinna from the Perna-bed of Atherfield, Isle of Wight, preserved in the Sedgwick Museum at Cambridge, has form and proportions closely similar to those of P. atherstoni. The specimen is unfortunately ill-preserved and unfitted for a detailed comparison. A similarly broad form has also been recorded from the Buda Limestone, the upper part of the Lower Cretaceous series of Texas.*

GENUS OSTREA Linnæus.

OSTREA SP.

Specimens of Ostrea (305, 306, 336) which occur in the cliff below the old school-house at Dunbrodie (right bank of Sunday's River), in association with Perna atherstoni, Pecten cottaldinus and Gastrochana dominicalis, are in a very poor state of preservation and consist chiefly of fragments. Valves of flattened form measure upwards of 40 mm. from the umbo to the opposite border and about 30 mm. from margin to margin in a direction perpendicular to this. The outline of the valve becomes broadly pointed at the umbonal end. Other imperfect individuals are represented by valves of smaller dimensions, and these appear to be less elongated and more None of the valves suffices for specific determination, but it is probable that two forms are represented.

convex.

GENUS EXOGYRA T. Say.

EXOGYRA IMBRICATA Krauss.

1843. Exogyra imbricata F. Krauss, Amtlicher Bericht über die zwanzigste Versammlung der Gesellsch. deutsch. Naturforscher und Aerzte zu Mainz im Sept., 1842, p. 129.

1850. Exogyra imbricata F. Krauss, Nov. Act. Acad. Cæs. Leop.Carol. Nat. Cur., vol. xxii., pt. 2, p. 460, Tab. 50, figs. 2a-2d.

1856. Gryphaa imbricata D. Sharpe, Trans. Geol. Soc. Lond., ser. 2, vol. vii., p. 197, pl. xxiii., figs. 3a, 3b.

* Shattuck (1), p. 19, pl. vii.

1867. Ostrea imbricata R. Tate, Quart. Journ. Geol. Soc., vol. xxiii., p. 154.

1904. Pycnodonta imbricata H. Douvillé, Bull. Soc. Géol. France, ser. 4, tome iv., p. 215.

Occurrence. This form is widely and abundantly distributed in the Marine Beds of the Zwartkop's, Sunday's, and Coega River valleys. It was met with at almost all the exposures in the Zwartkop's valley visited by Messrs. Rogers and Schwarz. amples sent to me are from the railway cutting between milestones 24-24 on the railway from Uitenhage to Graaff-Reinet (312), and others were collected by Miss M. Wilman at Coega. Specimens sent from the collection of the South African Museum are from the Sunday's River (300, 301, 302, 303).

Remarks. A striking character of the late adult and senile condition of E. imbricata consists in the prodigal manner in which shell substance is added at the margins of the valves, particularly noticeable in the smaller (right) valve in the neighbourhood of the hinge. Here, with its margin consisting of many coarse, imbricating laminæ, this valve may attain a thickness exceeding 30 mm. The ligament pit is seen to be strongly curved near the apex, but straight in the portion representing the later growth-stages.

This abundant and characteristic shell has given rise to some differences of opinion concerning its generic position within the Ostreide, which, however, may be sufficiently accounted for by the great variability of form exhibited by different individuals, and by the fact that the shell presents features which are recognised as leading characteristics of both Exogyra and Gryphæa. For this reason Tate used the name Ostrea in the broad sense, stating his opinion that the species in question "affords a good illustration of the mere sectional value (which cannot at all times be employed) of the groups Exogyra and Gryphaa." In many instances, it must be admitted, apparent passage forms seem to render difficult the definite separation of the three genera, so easily distinguishable when represented in their more characteristic aspects.* Jackson has made some luminous observations on the relation of these three genera and on the influence of the character of attachment by cementation in producing modifications in members of the Ostreidæ.+ He believes that Gryphea and Exogyra "are probably aberrant members of the Ostreidæ, not typical forms in the line of evolution of the group."

Leymerie (la). See also Peron's remarks on these genera, Peron (1), pp. 107-9. † Jackson (1), sections v. and vii.

It is highly probable that the shells classed as Gryphaa do not represent a homogenetic group, but are polyphyletic in origin, including repeated offshoots from an ostrean stock. While, in the progress from Ostrea to Gryphea, there is diminished duration of the attached habit, so that the typical Gryphaa is fixed by cementation of the left valve only during the young stage, examples are not wanting in which the highly specialised characters of Gryphaa are seen to become modified by the acquirement once again of more prolonged attachment. As an instance of this, reference may be made to the shells which, at the close of the deposition of the Oxford clay in England, seem to have largely replaced the familiar Gryphæa dilatata J. Sow. In these modified forms, the attached. valve did not become free until the neanic stage had been completed or the adult stage had been well entered upon, and the shell therefore perforce retained a relatively flatter and more ostreiform aspect. Exogyra imbricata might be thought, on cursory examination, to bear no slight analogy to these: the area of attachment has very frequently a similar relation, in point of dimensions, to the whole fully-grown valve, and it is only on the cessation of attachment that the individual acquires the manner of growth of a Gryphea and develops the arcuate form which led Sharpe to institute comparison with Liassic shells. It must be realised, however, that in this African form we have an illustration of the passage from a more complete to a less persistent duration of attachment, in the life of the individual, for a study of the youthful stage shows beyond doubt the exogyrate ancestry, and this may be clearly seen in many specimens in which the nature of the youthful characters did not become masked by the modifications incidental to fixation. The duration of attachment varied very much in different individuals, and no doubt often depended upon the nature and form of the object to which the young shell adhered. In some cases a relatively large area of attachment may retain the impress of some foreign surface, such as that of the large and coarsely ribbed Cucullaa kraussi Tate, or a pseudo-quadrate Trigonia, in a manner which largely obscures the true nature of the umbonal region. Other individuals, again, seem to have secured themselves to some less suitable or stable surface, and at an earlier stage to have entered upon the period of freedom. In these, as in some which have been attached to an even surface, the characters of the fixed stages are clearly seen to be those of a true Exogyra, and present the strongest contrast to Compare also figure of Gryphaa alligata from the Corallian of Nattheim Quenstedt (1), p. 752, Tab. 91, fig. 25 (1857).

*