190 Annals of the South African Museum. 



The description of this species given by Hall " is as follows : — 



" Shell somewhat semielliptical, varying to suborbicular or trans- 

 versely oval, generally broader than long. Ventral [pedicle] valve 

 convex, most prominent along the middle, declining laterally ; beak 

 incurved, with a small round perforation in the extremity which is 

 completed on the lower side by two deltidial pieces ; or, in the 

 absence of these the foramen is completed by the umbo of the 

 dorsal valve. 



Dorsal [brachial] valve flat ; beak straight ; hinge sloping from the 

 beaks at an angle of 110° to 160°, rounded at the extremities. 



Surface marked by ten to fourteen simple angular plications ; two 

 of which, on the middle of the ventral valve, are a little larger and 

 slightly more prominent than the others ; and between these there is 

 a third smaller depressed plication, forming an indistinct sinus. On 

 the dorsal valve the two middle plications are a little closer together, 

 and slightly more prominent near the front, than the others ; while 

 the depressions separating them from these, each side, are a little 

 wider and deeper than those between the other plications. . . . This 

 species presents some varieties of form, apparently due to age and 

 other influences ; the hinge-line of some individuals being often more 

 extended and nearly straight. The extension of the beak varies in 

 different individuals, and in those from diliferent localities." 



This species is well represented in the collection by numerous indi- 

 viduals, and in some localities (No. 221) it occurs in crowds. It varies 

 somewhat in its surface-ornamentation and shape, as Hall remarks, 

 particularly in the strength of the median plications. In the 

 majority of individuals the two plications bordering the median sinus 

 on the pedicle valve are stronger than the lateral plications, and in- 

 crease in height towards the anterior margin, while the correspond- 

 ing pair of plications on the brachial valve are likewise specially 

 strong and elevated anteriorly. The plications on the pedicle valve 

 curve slightly outwards on each side, and the median fold in the 

 sinus is always distinct though small. The number of plications is 

 usually ten. But none of the differences are of importance, and the 

 more coarsely plicated American individuals such as Hall illustrates 

 {op. cit., vol. iii., pi. ciii. B, fig. 1 a-f) exactly resemble the Cape 

 specimens. The younger individuals are less strongly plicated, and 

 the folds are more uniform in size. 



It was this species which Morris and Sharpe described as Atnjpa 



* Hall, Palfeont. N.Y., vol. iii., 1859, p. 449, pi. ciii. B, fig. 1 a-f; pi. cvi., 

 fig. 1 a-f. 



