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. Examples 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 Gryphaa. 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.t 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 (1a). See also Peron's remarks on these genera, Peron (1), † Jackson (1), sections v. and vii. pp. 107-9. 1 It is highly probable that the shells classed as Gryphæa 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 Gryphea 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 Gryphea 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 Gryphæa alligata from the Corallian of Nattheim Quenstedt (1), p. 752, Tab. 91, fig. 25 (1857). * the features of the nepionic and neanic stages of Gryphea as illustrated, for instance, in the admirable descriptions and figures given by Hill and Vaughan of members of the genus occurring in the Lower Cretaceous strata of Texas. When we see how closely the manner of growth in the adult stage of E. imbricata often simulates that of a true Gryphaa with simple ostrean ancestry, it must be admitted that the parallelism is very striking. It would be difficult to indicate within narrow limits the true relationship of Exogyra imbricata, but it is evident that its nearest morphic counterparts are essentially characteristic of Lower Cretaceous rocks. Thus, a very close resemblance exists between this form and some of the shells classed by Leymerie under the name Exogyra sinuata and E. subsinuata. Leymerie first regarded all these as varieties of Exogyra sinuata,† and included forms to which numerous specific names had previously been applied. He subsequently separated E. subsinuata as a species distinct from E. sinuata, and used several varietal designations of both. Regarding the value of the nomenclature employed by this author or by Coquand‡ in dealing with the forms comprised within this perplexing group, I am at present unable to attempt the critical and detailed study which alone would warrant the expression of any judgment; in like manner, the varying use of the name Exogyra couloni by different authors has given rise to such a degree of confusion that, in instituting comparisons, it must suffice to make reference to actual figures or descriptions, leaving aside for the time being all question of the value or correctness of the specific names attached to certain of these. Krauss rightly recognised the broad relationships of E. imbricata and believed it to be nearly related to "Gryphaa couloni Defr.," from which, however, he found it to differ in the following points: "It is larger, more thick-shelled, longer and narrower; has a keel narrower at the end (though always rounded), more concave laminæ lying much higher one upon the other, and a stronger umbo bent further over the opercular valve, than in Gryphæa couloni Defr." Some of the shells figured by d'Orbigny as Ostrea couloni § show no slight resemblance to selected individuals of E. imbricata, but are narrower than the generality of the Uitenhage specimens, and the imbricating lamella of the larger valve are more prominent and irregular. The broad, plicated specimens figured by d'Orbigny under the same name on plate 466 of his work cannot be brought * Hill and Vaughan (1). ‡ Coquand (2). ↑ Leymerie (1). § d'Orbigny (3), pl. 467, figs. 1–3 (1848). into comparison. The shells figured by Coquand* as Ostrea couloni include narrow forms, more elongated and more decidedly carinated than E. imbricata, as well as a broad type, developing plications of the larger valve in the later adult stage, which is in no respect closely comparable with the form we are considering. Sowerby's Exogyra sinuata, though itself very variable, is typically widely separated from E. imbricata by the strong carination of the larger valve and the widely expanding form of this, with the much less arcuate habit of growth and the considerably greater adult dimensions. Some French forms figured by Leymerie and brought into relationship with Sowerby's species, though perhaps erroneously, afford, on the other hand, material for a close comparison with E. imbricata. Thus, Leymerie's E. sinuata, var. elongata can be fairly closely matched, while extreme individuals of E. imbricata, in their narrow outline and very arcuate growth, nearly resemble that author's figure of E. subsinuata, var. aquilina.§ The shell from the Neocomian of the Haute-Marne figured by Bayle || as Aetostreon aquilinum (Leym.), again, closely resembles very narrow and curved specimens of the African form. Ostrea tardensis Stanton,¶ from the Lower Cretaceous Gio beds of Patagonia, may, in selected individuals, somewhat closely approach E. imbricata, but its near relationship to this must be considered doubtful; it seems, however, to represent in some respects an analogous type. The exogyrate character of the umbonal region is only feebly indicated, according to Dr. Stanton, who says that the lower valve is very thick and very convex, obscurely carinate, with the beak more or less twisted laterally, but not distinctly coiled." It differs also by frequently acquiring a marked crescentic form in the adult. If the Patagonian shell had true exogyrate ancestry, which appears probable, the early characters seem to have so far disappeared as to indicate that relationship to E. imbricata is probably somewhat remote. The shell from the Neocomian of Arqueros in Chili described by Bayle and Coquand as Ostrea couloni,** and afterwards brought into association with a heterogeneous company under the collective name * Coquand (2), p. 180, pl. 65, fig. 10; pl. 71, figs. 8-10; pl. 74, figs. 1-5; pl. 75, figs. 1-6, 22. + J. Sowerby (1), vol. iv., Tab. 336 (1822). [As Gryphæa.] Leymerie (2), p. 17, pl. 12, fig. 2. § Ibid., pl. 12, figs. 6, 7. Bayle (1), pl. cxl., figs. 3-5. ¶ Stanton (3), p. 11, pl. i., figs. 1, 2; pl. ii., figs. 1, 2. "Ostrea aquilina" by Coquand, closely resembles many specimens of E. imbricata in the form and inflation of the valves and in the degree of curvature of the umbonal region; but it differs by the more marked carination of the larger valve. Remes has considered both the Chilian shell and Exogyra imbricata Krauss to be identical with Sowerby's Exogyra sinuata, but this is certainly an error. The same author, however, has figured an Exogyra from Stramberg,‡ ascribed by him to Exogyra subsinuata Leym., to which selected individuals of E. imbricata, in which the surface of attachment is relatively large, may show considerable resemblance. An exogyrate shell from the Bajocian of Abyssinia was ascribed by Douville § to E. imbricata Krauss, but this was an erroneous identification, as already pointed out by G. Müller. The resemblance of the small specimens figured by Douvillé to selected young individuals of E. imbricata is indeed not a distant one, but many of the Uitenhage specimens are attached by a very much larger surface, and, moreover, attain a colossal size in comparison with Douville's types, if these represent the adult stage. The variation in E. imbricata is so great that a more detailed and critical comparison could not be entered upon without a substantial suite of specimens of the Abyssinian shell. A shell from the Isakondry basin in Madagascar (presumably from strata of Lower Cretaceous age) is stated by Douvillé to bear a close resemblance to Exogyra imbricata Krauss.¶ GENUS MYTILUS Linnæus. MYTILUS UITENHAGENSIS sp. nov. Plate II., figs. 10, 11, 11a. Description. The shell is slender and elongated in outline, sharply pointed in front, and antero-ventrally truncated. The hinge-margin is straight, and passes backwardly by a curve into the obliquely sloping posterior margin, which is only very slightly convex in outline until the posterior end of the valve is reached. In profile, the antero-ventral margin is straight, passing from the acutely pointed umbo to the posterior angle of the valve, where it abruptly cuts the curved outline of the posterior margin. The antero-ventral area is flat, and its surface is perpendicular * Coquand (2), p. 158. Remes (1), pl. xxi. (iv.), figs. 2a, 2b. § Douvillé (1), p. 230, pl. xii., figs. 8, 9. ¶ Douvillé (2), p. 388; Douvillé (4), p. 215; † Remes (1), p. 216. G. Müller (1), pp. 569, 570. |