3 3.5 3.5 4 Greatest depth of a single valve Occurrence. Found in the kloof east-north-east of Red House, on the left side of the Zwartkop's River (324), and at Grass Ridge, three miles east-north-east of Uitenhage (310); also in the railway cutting between milestones 241-24 on the line from Uitenhage to GraaffReinet, about three miles from Uitenhage (316). A specimen sent from the South African Museum is from the Sunday's River, occurring with Holcostephanus cf. atherstoni (Sharpe). Mr. Rogers obtained specimens of this form, in 1905, from the left side of Coega Valley, half a mile down from the railway (453g, 454g); from a bare slope, W. 30 S. from the middle of Barkly Bridge, on the farm Olifant's Kop, Sunday's River (24h, 26h, 28h-30h); and from the highest beds in the kloof behind Colchester, left bank of Sunday's River (493g, 498g). Remarks. Although this elegant shell shows some variation in outline and in the degree of inflation, yet the individuals here brought together agree on the whole very closely and are well characterised. The chief features of the shell are the generally rounded and soft outline, the inconspicuous, rounded umbonal region, and the very compressed form of the valves. In outline, the shell resembles some specimens of Venus orbignyana Forbes, from the Lower Greensand of the Isle of Wight, which differs, however, by the considerably stronger inflation and the more marked anterior excavation under the umbones. Meretrix uitenhagensis has a less steeply sloping upper margin posteriorly to the umbones, and the umbones are less prominent. The minute concentric linear striæ are developed also in Venus orbignyana, though more faintly marked than in the African shell. Meretrix parva (Sow.), also from the Lower Greensand of England, is more circular in outline, more nearly equilateral, and considerably more inflated in form. Meretrix brongniarti (Leymerie) is a larger and more massive shell, and even if compared at the same dimensions is seen to be more elongated and posteriorly attenuated in outline, and more equilateral. * Forbes (1), p. 240, pl. ii., fig. 5. † J. de C. Sowerby (1), vol. vi., Tab. 518, figs. 4-6 (1826). Meretrix labadyei (d'Archiac),* from the Tourtia of Tournay, has an almost identical outline, but its valves are much more convex. The same character distinguishes several other Cretaceous forms ascribed to Cytherea or Venus, which, when compared with our shell, are seen to have a very similar outline. In the shape of the shell, the inconspicuous umbones, and the compressed form, resemblance is shown to Tapes picteti de Loriol,t from the Gault of Cosne (Nièvre), which, however, is not so elongated relatively to height, and moreover, has a coarser concentric ornamentation. A similar resemblance is shown to Tapes patagonica Stanton, from the Belgrano beds (Lower Cretaceous) of Patagonia, but the points of distinction are plainly seen in the less elongated figure, the greater inflation, and the coarser concentric ornamentation of the Patagonian shell. A shell from supposed Lower Cretaceous strata in the Cameroons (left bank of Mungo River), described by von Koenen § as Cytherea wohltmanni, differs from Meretrix uitenhagensis in the greater inflation, the more inequilateral form, and the prominence of the umbonal region. A word may be added regarding the generic position of this form. In no specimen has the interior been seen, so that a precise generic determination cannot really be made with certainty; but a comparison with other species in which the hinge-teeth are known, justifies a provisional reference to Meretrix, if this name be applied in the broad sense in which the name Cytherea has for long been used, with reference to Cretaceous forms. The tendency of modern work is to set closer and closer limits to the application of longestablished generic names amongst lamellibranchs, as in other classes of Mollusca. It is highly probable, when evidence of internal characters can be obtained, that an extension of this principle to Cretaceous forms, on the lines carried out in the classification of recent and Tertiary species, may eventually show the inapplicability of the name Meretrix (equivalent to Cytherea as commonly used) to such a form as the one here described. From practical considerations, however, it will often be necessary, as in the present case, to continue to utilise in a broad sense a name which, though perhaps technically wrong, conveys as definite a meaning as the available evidence for the time being allows. * d'Archiac (1), p. 303, pl. xiv., fig. 7. Stanton (3), p. 23, pl. iv., figs. 12, 13. † de Loriol (4), p. 64, pl. vii., fig. 21. § von Koenen (1), p. 36, Taf. iv., figs. 6, 8, 9; since shown to be of Upper Cretaceous age, see Solger (1). GENUS PSAMMOBIA Lamarck. PSAMMOBIA ATHERSTONI Sharpe. 1856. Psammobia atherstoni D. Sharpe, Trans. Geol. Soc. Lond., ser. 2, vol. vii., p. 196, pl. xxii., fig. 11. This was found in the cliff below the old school-house at Dunbrodie, on the right bank of Sunday's River (321, 325). A specimen from this locality, submitted to me, agrees very closely with the largest individual figured by Sharpe. The Dunbrodie specimen measures 25 mm. in length and 15 mm. in height; it is a left valve, having the characteristic compressed form and flattened flank and very inconspicuous umbo. Sharpe records this shell from the Sunday River near Enon, in a grit sometimes full of the casts of the shell." At Dunbrodie Psammobia atherstoni is associated with Acteonina atherstoni, Turbo atherstoni, Pecten cottaldinus, and other forms. Messrs. Rogers and Schwarz mentioned the occurrence of Psammobia atherstoni in the Wood Bed series of the Bezuidenhout's River below Blue Cliff, but this name was probably applied to specimens which I have referred to the genus Unio. 66 The generic determination of this species must be regarded as provisional. No specimens showing satisfactorily the internal characters have been available for study, and true relationship with Psammobia (or Gari Schumacher, if this be regarded to have synonymic value) still remains to be proved. The propriety of considering the name Gari to possess the same significance as that which has been ascribed to Psammobia (sensu lato) is perhaps still open to question. The shells we are dealing with in the present instance are smooth and without radial markings, and this fact, together with the slight doubt concerning even the broader generic relationships, seems to justify the retention, for the time being, of Lamarck's more familiar and more widely accepted name. It appears reasonable, under the circumstances, to use as a provisional measure a nomenclature which clearly indicates the supposed relationships, though it may perhaps be technically erroneous. То hazard a "correction" of nomenclature on an insecure basis of imperfect knowledge is a step for which it would probably be more difficult to find justification. GENUS SOLECURTUS H. D. de Blainville. Text-figure 1. Description of a Single Specimen.-The specimen is a right valve, slightly imperfect at the siphonal margin. The umbo is very inconspicuous, and is situated slightly in front of the middle of the shell. The upper margin slopes down very gently in front and behind the umbo. The frontal margin is short and has a rounded convex profile; towards this margin the upper and lower valve-borders very gently converge. Towards the siphonal margin the valve has a slightly greater height than in the anterior half. The surface is marked by lines of accretion, but no trace of ornamentation has been observed. The inflation of the valve is very slight. Occurrence. Found at Grass Ridge, 3 miles east-north-east of Uitenhage (310). * Remarks. The single specimen examined is not quite complete at the posterior end, and, unfortunately, the surface is so preserved as to leave no traces of faint linear sculpture, if such markings were ever present. By following the growth-lines it is possible to arrive at the original form of the posterior border. In shape the shell agrees very closely with Solecurtus warburtoni Forbes, from the Aptian of Atherfield (Isle of Wight); the umbo occupies a similar position, and the valve is anteriorly rounded, with diminished height towards the frontal border, while posteriorly the height is greater. The only clearly observable difference is that, in specimens of similar dimensions, the measurement from the umbo vertically to the inferior margin is relatively rather smaller in the English shell. * Forbes (1), p. 237, pl. ii., fig. 1. This also has very faint and delicate radial, linear ornamentation on the anterior part of the flank, but the African shell may originally have borne similar minute sculpture, and on this point nothing further can at present be said. Similar forms were described by d'Orbigny, under the name Solen, from the Lower Cretaceous (S. robinaldinus)* and from the Chalk (S. æqualis).† Guéranger has figured a shell from the Cenomanian of the Sarthe under the name Solecurtus æqualis,‡ to which our specimen appears to bear a close resemblance, so far as comparison is possible. Description. The shell has greater length than height, and the umbones are situated at rather more than one-third of the shell's total length from the anterior extremity. The inflation is moderate, and most strong just above the middle of the valves. The umbones are somewhat weakly developed and are not very prominent. The cardinal margin and the posterior margin form together a curved outline which passes down, posteriorly very steeply, to a marked angular junction with the lower border. From the umbo a wellmarked carina passes obliquely across the posterior part of the valve, down to the postero-inferior angle of the valve-margin. The carina marks off a flattened or very slightly concave postero-superior area which occupies less than one-quarter of the total valve-surface. In front of the umbo the valve-margin slopes down to form a somewhat sharply curved outline in front, the foremost point of the anterior margin falling well within the lower half of the shell. The long pallial margin shows a broadly convex outline, sometimes slightly flattened towards the posterior end. The greatest height of the valve occurs at the umbonal part. The surface is smooth, with numerous delicate growth-lines, and shows closely spaced radial rows of very minute punctæ, only visible under a lens. |