PjECILASMA eburnea. 1 13 



placed almost transversely to the peduncle. Valves 

 white, smooth, moderately thick. 



Scuta: the basal margin, as seen externally, is narrow, 

 and can hardly be separated from the carinal margin ; 

 but an internal basal rim, (fig. 5, b) (along which the 

 imbedded disc of the carina runs,) shows where, in 

 the other species, the basal and carinal margins are 

 separated. This basal internal rim is not parallel to the 

 external basal margin, but runs upwards to the occludent 

 margin, leaving beneath it a large triangular space, to 

 which the membrane of the peduncle is attached ; and 

 this makes it appear as if the rostral umbones of these 

 valves had grown downwards ; but, judging from the 

 allied species, P.fissa, I have no doubt that the pri- 

 mordial valves really lie on the umbones, and that the 

 growth has been in the usual direction, that is, exclu- 

 sively upwards. The occludent margin is curved, and 

 blends by a regular sweep into the carinal margin, so 

 that there is no acute upper angle. A distinct line can 

 be seen, as if two calcareous valves had been united, 

 running from the umbo to the upper end of the valve, 

 thus in appearance separating a slip of the occludent 

 margin ; internally this appearance is more conspicuous ; 

 this structure is important in relation to that of P. fissa. 

 The pointed umbones are divergent, and internally under 

 each, there is a large tooth. The two valves are equally 

 convex. 



Terga, entirely absent. 



The Carina (Tab. II, fig. 5, a, c), including the disc, 

 is three fourths as long as the scuta ; it is placed almost 

 transversely to the longitudinal axis of the peduncle ; it is 

 narrow and internally convex ; the imbedded disc is very 

 large, forming a continuous curve with the upper part of 

 the carina j this disc runs along the internal basal rim of 

 the scuta, and hence almost separates, internally, the 

 peduncle from the capitulum ; it equals one fourth of the 

 total length of the valve, and is thrice as wide as the 

 upper part; it is oval, externally marked by a central 



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