158 Annals of the South African Museum. 



preserved the surface is seen to be covered with closely spaced 

 radial lines of delicate granules. 



Although possessing no highly distinctive external features, this 

 shell retains a certain characteristic aspect, while approaching some- 

 what closely to some European representatives of the genus. It 

 may usually be easily distinguished from the larger Pleuromya 

 lutraria (Krauss),* another characteristic shell of the Sunday's 

 River Beds, with which it is associated. If mature specimens be 

 compared, P. lutraria is distinguished by its much larger dimen- 

 sions. If small specimens of this be brought into comparison with 

 mature examples of P. baini of equal size, there is a considerable 

 general similarity between them ; but P. lutraria, when young, has 

 a definite ornamentation of rounded concentric ribs, whereas P. baini 

 is a smooth shell, with the surface only marked by lines of growth 

 and minute granules. The umbones of P. lutraria are rather more 

 prominent, and are perhaps situated a little nearer to the anterior 

 extremity, while the margin in front of the umbo slopes down more 

 suddenly than in P. baini. 



Pleuromya neocomiensis (d'Orb.) t is a similarly elongated form, 

 but is distinguished by marked anterior truncation. P. rostrata 

 (d'Orb.) I is more produced posteriorly and has a more curved 

 inferior outline. Some individuals of P. baini closely resemble a 

 Pleuromya from the Lower Greensand of the Isle of Wight which 

 has been considered, apparently erroneously, to represent d'Orbigny's 

 P. neocomiensis, and is much more likely to be identical with 

 P. schroderi (Wollemann), from the Neocomian of North Germany. 

 To judge from Dr. Wollemann's description and figure, P. schroderi 

 approaches very closely indeed to P. baini. 



Another shell which shows considerable resemblance to P. baini 

 occurs in the Neocomian sandstone of the Teutoburger Wald, and 

 was ascribed by Weerth,|| though perhaps erroneously, to d'Orbigny's 

 Panopcza neocomiensis ; the general similarity is great, but the shell 

 figured by Weerth has an inferior margin presenting a rather more 

 curved outline. 



The shell from the Neocomian of North Germany, described by 

 F. A. Boemer 11 as Pleuromya solenoides, is an elongated form with 



* Krauss (2), p. 447, Tab. xlvii., fig. 1. 



t d'Orbigny (3), p. 329, pi. 353, figs. 3-8 (1845). 



} d'Orbigny (3), p. 333, pi. 355, figs. 3, 4 (1845). 



Wollemann (1), p. 126, Taf. v., fig. 7 (as Panopcea). 



\\ Weerth (1), p. 37, Taf. viii., fig. 7. 



T F. A. Eoemer (3), p. 330, pi. xli., figs. 20 21. 



