Cf. 1847. Pecten subacutus A. d'Orbigny, Paléont. Franç., Terr. Crét., vol. iii., p. 605, pl. 435, figs. 5-10. 1872. Pecten subacutus H. B. Geinitz, Das Elbthalgeb. in Sachsen, part i., p. 195, pl. xliv., fig. 5 (Palæontographica, vol. xx.). 1902. Pecten (Chlamys) subacutus H. Woods, Mon. Cret. Lamell. Engl., vol. i., part iv., p. 169, pl. xxxi., figs. 7-9. (Palæontographical Society.) Descriptive Note.-A single specimen sent from the collection of the South African Museum seems to approach very closely to P. subacutus. The shell is ovate in outline, prolonged dorsally, with an acute apical angle (about 80°). The inflation is slight, with the left valve a little more convex than the right. On The valves are ornamented by about 22 strong, angular ribs with sharp summits. Near the inferior margin the summits of the ribs are separated from one another by a distance of about 2 mm. parts of the shell the ribs are studded with numerous blunt, spinose projections. Where the surface is well preserved the ribs and interspaces are seen to be ornamented by very delicate, transverse linear markings, most clearly visible in the interspaces. Near the inferior margin these become obscurely developed and are masked by coarser ridges and grooves of accretion. The hinge-line and ears are unfortunately not preserved, but having regard to the other characters there is no reason to suppose that they differed essentially from those of P. subacutus. Dimensions.-Height (about) 42 mm.; length 35 mm. Occurrence.-Marine Beds of Sunday's River (304). Remarks.-Pecten subacutus is characteristic of the Cenomanian in Europe, but this African shell approaches more closely to it than to any form from the Lower Cretaceous with which I am acquainted. In the imperfect material at disposal, no features are apparent which would justify a definite separation from the European form, yet in the absence of the ears in this specimen and the lack of further comparative material, the question of identity must remain open. Pecten urgonensis de Loriol,* common in the Urgonian of Grand Salève, is a shell of very similar type, so far as can be judged from de Loriol's description and figures; but apparently its scaly ornaments are larger and more prominent, and the surface is perhaps more coarsely ribbed. * de Loriol (2), p. 389, pl. C, figs. 25, 26. GENUS LIMA J. G. Bruguière. LIMA (ACESTA) OBLIQUISSIMA Tate. 1867. Lima obliquissima R. Tate, Quart. Journ. Geol. Soc., vol. xxiii., p. 156, pl. ix., fig. 5. Supplementary Descriptive Note.-The shell has great height in comparison with length. The posterior outline is gently rounded, the anterior profile straight. The straight anterior border passes at its lower end by a somewhat sudden curve into the rounded inferior border. The valves are most convex in the umbonal half and near the anterior side; they become flatter posteriorly and inferiorly. Anteriorly, the flank passes abruptly, though without marked carination, into an extensive, flat frontal face, truncating the shell. This frontal area is in no degree sunk or concave in form. The umbones are acute and sharp, terminal at the anterior extremity of the hinge-line; the superior border of the shell is straight and very short, and truncates the shell to form a right angle with the anterior border. The anterior ear is rudimentary or not developed ; the posterior ear is much reduced, and not definitely demarcated from the flank. The ornamentation consists of delicate linear radial grooves, very crowded near the umbo, gradually diverging until, at a distance of 40 mm. from the umbo, they may be separated by interspaces 2 mm. broad. At a distance exceeding 30 mm. from the umbo, these lines are almost or quite absent from the middle part of the flank, though well impressed on the lateral parts. The lines follow a more or less irregular and wavy course and are sometimes suddenly deflected to right or left when traced down from a prominent concentric growth-line. When well preserved, the linear ornaments show a minutely punctate structure, most perfectly developed in the umbonal half of an adult shell. The interspaces are very gently convex or are flat, but are without sculpture. On the frontal area the ornaments are of a more pronounced character, and may take the form of imbricating ridges with the edges directed away from the valve-margin. They radiate from the umbonal region and pass along the frontal face, gradually approaching the frontal valve-margin and forming an acute angle with this. On the frontal area, therefore, the ornaments are very obliquely crossed by the lines of accretion. Dimensions.— Greatest measurement radially from the umbo ... 64 mm. the last Greatest depth of a single valve .... 40,, 10,, Occurrence. Railway cutting between milestones 241-24 on the line from Uitenhage to Graaff-Reinet, about three miles from Uitenhage (346); also stated to occur at Grass Ridge. Specimens in the collection of the Geological Society are labelled "Sunday River" (Rubidge) and "Zwartkop River" (H. Longlands). Tate's record of locality is "in a yellow shell-rock from the Zwartkop River sandstone, with Placunopsis undulata and fossil wood (Rubidge)." Fine specimens sent to me from the South African Museum are from the Sunday's River. Mr. Rogers obtained this form on the left side of the Coega Valley, half a mile down from the railway (455g). Remarks.-Tate unhappily brought this shell into comparison with two British Jurassic forms, L. rigidula (Phill.) from the Cornbrash, and L. ovalis (J. Sow.) from the Great Oolite. Lima rigidula* is of quite another type; it has a widely different outline, is comparatively coarsely ornamented, and has well-developed anterior and posterior ears. L. ovalis,t more oblique and anteriorly produced, with minutely delicate and crowded linear ornaments, can scarcely be brought into close comparison. Lima obliquissima in reality may be most closely compared with those shells to which has been applied the sub-generic name Acesta,‡ represented in Cretaceous and later strata, and existing at the present day. The characters whereby this group of forms is distinguished from Plagiostoma and other divisions of the genus have been clearly set forth by E. Philippi in his analysis of Lima.§ Although I have as yet been unable to ascertain the position and form of the ligament pit in L. obliquissima, the close agreement in other features can leave little or no doubt that this shell must be united with Acesta, if this sub-generic group be adopted at all. These features are principally seen in the great relative height of the valve; the anteriorly little-produced outline; the anterior and terminal position of the umbones; the great reduction of the anterior ear and the imperfectly demarcated posterior ear. The fine linear * Phillips (1), p. 116, pl. vii., fig. 13.` + J. Sowerby (1), tab. 114, fig. 3 (1815); Morris and Lycett (1), Part 2, p. 29, pl. iii., fig. 5. H. and A. Adams (1), p. 558. 8 E. Philippi (1), p. 630. sculpture of the flank, it is true, recalls that of some Plagiostoma, but the shell is in other respects strongly contrasted with that group. Concerning the value of the sub-generic name Acesta, particularly in the application to fossil forms, opinions have widely differed. It was accepted by Stoliczka and by Zittel,† and latterly by Philippi, while regarded by Fischer as only representing a section of Lima. This last author merely quotes the living type species, L. excavata. G. Boehm, however, has expressed the opinion that the sub-genus Acesta cannot be upheld,§ pointing out that the original diagnosis is inaccurate; that the name is unpractical in its application to fossil forms, in many of which the direction and position of the ligament pit, upon which the exact determination depends, cannot be investigated; and further, that the distinctive features of the proposed sub-genus are of slight importance. The first of these objections can have little weight, merely depending, as pointed out by Philippi, upon an original error of orientation. There is something to be said for the last point, perhaps, and it is clear that amongst fossil representatives of the genus there are some forms which illustrate in varying degrees the inequilateral character and the development of the anterior auricle. The name, however, though possibly not standing for a natural group, may for the present be conveniently employed to indicate the apparent affinities of the forms to which it is applied. L. obliquissima shows a striking outward similarity to the typical living Lima (Acesta) excavata (Fabr.) Chemn.,|| though differing from this by the relatively more elongated outline, the shorter hingeline, and the more restricted posterior cardinal expansion of the flank. A Neocomian shell having great resemblance in form to L. obliquissima is Lima undata (Desh.) Leym.,¶ but it is distinguished by its longer hinge-line and stronger ornaments. In the shortness of the superior margin and the character of the surface ornamentation, Lima orbignyana Math.** shows closer agreement, but it is distinguished from L. obliquissima by the more perfectly demarcated posterior ear, the concave outline behind this, the concave anterior area and the extension of linear ornaments over the whole flank. Matheron's figure shows a distinctly developed projecting anterior * Stoliczka (2), pp. 413–415. Fischer (1), p. 941 (1886). || E. Philippi (1), Taf. xxiv., fig. 5. ¶ Leymerie (2), p. 10, pl. 8, fig. 8; (1847). + Zittel (5), p. 26 (1881). 8 G. Boehm (1), p. 625. d'Orbigny (3), p. 528, pl. 414, figs. 9-12 ** Matheron (1), p. 182, pl. 29, figs. 3, 4 auricle, though this is represented as much more reduced in the figure given by d'Orbigny. A Lima from the Lower Cretaceous of England, ascribed by W. Keeping to Roemer's L. longa, shows great similarity in general form and outline. It differs, however, from the African shell by the relatively longer hinge-line and more extensive posterior ear, and although appearing to vary considerably in regard to the surface ornamentation, is characterised by a more pronounced type of radial sculpture; L. longa is further distinguished by the possession of great height measurement in proportion to its length.t There is closer agreement, again, between L. obliquissima and L. tenuitesta Whitfield, from Upper Cretaceous rocks in Syria. The two are practically identical in outline and the relative length of the superior border, but L. tenuitesta has the linear striæ of the surface more numerous and closely crowded at a given distance from the umbonal apex. SUB-GENUS MANTELLUM J. F. Bolten. LIMA (MANTELLUM) NEGLECTA Tate. 1867. Lima neglecta R. Tate, Quart. Journ. Geol. Soc., vol. xxiii., p. 156, pl. vii., figs. 4a, 4b. Three specimens, all left valves, are referable to this species. The ribs are acutely ridged, with sharp summits. The middle of the angular interspace is sometimes occupied by a fine linear ridge, and occasionally a second one may be present. These are stronger than the numerous, delicate, linear ridges which are developed on the sloping sides of the main ribs. A specimen 20 mm. in length has a greatest diameter, measured at right angles to the length, of about 12 mm. Occurrence. On the left side of the Coega Valley, half a mile down from the railway (448g, 449g, 453g). The specimen described by Tate, numbered 11,013 in the collection of the Geological Society of London, came from the Sunday's River. Remarks.-Tate's figure of this species is very inadequate, and does not give a good idea of the shell. The appearance of two sets * Keeping (1), p. 112, pl. v., fig. 6. † F. A. Roemer (1), p. 79, Taf. xiii., fig. 11 (1836); (2), p. 57 (1841). Since the above lines were written, Mr. H. Woods has published an account of this shell and considers that the Upware specimens were rightly identified with the German form: Woods (3), vol. ii., p. 25 (1904). Whitfield (1), p. 390, pl. iv.a, figs. 1, 2. |