86 Annals of the South African Museum. 



developed as a rounded folding of the valve. Above this the surface 

 is evenly convex, while below it the valve is flattened. An un- 

 crushed specimen therefore shows in transverse section an outline 

 different from that depicted by Sharpe in his figure 36. It is true 

 that the upper portion of the valve also becomes much flattened in 

 the posterior part of an adult specimen, but only near the siphonal 

 margin. The ribs, starting above at the valve-margin, are directed 

 obliquely backwards, and before the lateral fold is reached they 

 bifurcate, at the same time curving sharply with the convexity back- 

 wardly directed. An occasional extra, curved rib is here intercalated. 

 Just below the lateral fold the ribs unite again in twos or threes and 

 pass forward in the form of a number of fine, unequally spaced 

 ridges which occupy the lower flattened portion of the valve and run 

 roughly parallel to one another and to the lower margin. In addi- 

 tion to these ridges, the valve below the oblique lateral fold is 

 ornamented by a series of very numerous and crowded, delicate, 

 vertically running raised stripes or wrinkles. This vertical wrink- 

 ling is very regular, and occupies the whole of the surface below the 

 lateral fold, crossing the horizontal ridges at right angles. The 

 wrinkles, which number upwards of forty within a space of 10 mm., 

 are developed both on the ridges and interspaces. 



Occurrence. Collected in the Zwartkop's Eiver valley, one mile 

 north-east of Eawson Bridge (281). Eecorded by Sharpe from the 

 Sunday's Eiver at " C. Eoe's drift," and by Stow from above Modder 

 Drift and from M'Loughlin's Bluff, on Sunday's Eiver. 



Remarks. The specimens figured by Sharpe, in the collection of 

 the Geological Society, show the vertical wrinklings on the lower 

 portion of the valve, though no mention w T as made of these. Well- 

 preserved specimens exhibit these structures very clearly, and show 

 that they form a definite part of the sculpture. 



The group to which this shell belongs is more characteristic of 

 Jurassic than of Cretaceous rocks. M. soiuerbiana (d'Orbigny) 

 ( = M. plicata Sow.),* a typical Oolitic form, has a much more 

 numerous division of the ribs above the lateral fold, and is rather 

 more curved in outline. M.perplicata (Etallon),f from the Upper 

 Jurassic of Europe, is similar in the bifurcation of its ribs ; but it 

 attains a greater height in relation to length, and has also a more 

 curved outline. The descriptions and figures of M. perplicata J 



* d'Orbigny (4), vol. i., pp. 282,312; J. Sowerby (1), vol.iii., Tab. 248, fig. 1 (1819). 

 f Thurmann and Etallon (1), p. 223, pi. xxix., fig. 8 (1862) ; Loriol and Pellat 

 (2), p. 156 (312), pi. xviii., figs. 19, 20. 



J See also Loriol, Eoyer, and Tombeck (1), p. 348, pi. xix., figs. 10, 11. 



