76 Annals of the South African Museum. 



variation. The convex fold of the valve, passing back from the 

 umbo, is marked off from a small, flattened, marginal portion of 

 the valve between the fold and the antero-inferior margin near 

 the umbonal end, in clearer manner as a rule than is depicted in 

 Sharpe's figure. In this respect the figure is misleading, for the 

 fold is well developed in the original specimen. In another point, 

 also, the illustration leaves something to be desired; the postero- 

 inferior border, restored by a dotted line, should in reality have been 

 represented as a rounded curve similar to that followed by the outline 

 of the same part in Perna mytiloides Lam., and should not have 

 been drawn in the angular or sub-angular form which lends such 

 a false aspect to the outline of the shell as depicted in Sharpe's 

 work. 



P. atherstoni is characterised by its oblique figure and the rela- 

 tively great length of the hinge-line. In these features it recalls 

 P. mytiloides Lam., from the Upper Oolites of England,* which in 

 general aspect it much resembles ; but in P. atherstoni the shell is 

 less sharply pointed and produced at the umbo, is less inflated 

 anteriorly, and is more obliquely elongated. There are further 

 points of difference which it is unnecessary to recount. 



GENUS PINNA Linnaeus. 

 PINNA ATHERSTONI Sharpe. 



1856. Pinna atherstoni D. Sharpe, Trans. Geol. Soc. Lond., ser. 2, 

 vol. vii., p. 193, pi. xxii., fig. 1. 



Occurrence. Eailway cutting between milestones 24|-24f on the 

 railway from Uitenhage to Graaff-Keinet, about three miles from 

 Uitenhage (329, 330), where it is very abundant in a bed of nodular 

 limestone. 



Remarks. It has been notedby Messrs. Eogers and Schwarzthat 

 this shell, associated with Holcostephanus atherstoni, marks the 

 most constantly recognisable zone in the Marine Beds of the Zwart- 

 kop's Kiver valley. 



Pinna atherstoni was compared by Sharpe and Tate with P. 

 hartmanni Zieten, from the Lias of Europe, and it certainly agrees 

 more closely with P. hartmanni, as figured by Goldfuss,f than with 

 the generality of Cretaceous forms, most of which are more elon- 

 gated and slender in outline. The original figure given by Zieten J 



* Damon (1), pi. ii., fig. 5. 



f Goldfuss (1), Band ii., p. 164, Tab. cxxvii., fig. 3b (1837). 



t Zieten (1), p. 73, Tab. 55, fig. 5 (1833). 



