1850.] 



MORRIS AND LYCETT ON PACHYRISMA. 



401 



valve being received into a small depression aboise the corresponding 

 process of the right valve^, the tooth of the right valve resting within 

 the muscular depression of the opposite one. The thickness of this 

 portion of the test is such, that in an individual which measured six 

 inches across, it was upwards of three-quarters of an inch. 



This shell has some affinities with Isocardia, Opis, and Megalodon ; 

 the latter of which it appears to represent in the Jurassic period, and 

 might with it constitute a family, ^' Megalonidce.'" It is distinguished 

 from Megalodon by the cardinal tooth in the right valve not having 

 been divided as in the latter genus. Megalodon has the anterior 

 muscular impression inserted on a somewhat raised or lamelliform 

 plate ; but it has however a slightly raised plate for the posterior 

 muscle, or rather the latter may be said to be bordered interiorly by 

 an obtuse angulated ridge. From Opis it is sufficiently distinguished 

 by the character of the dentition. The dichotomous ligament re- 

 sembles that of Isocardia, and when viewed anteriorly it reminds us 

 of the recent Isocardia cor with its large and graceful diverging um- 

 bones. Pachyrisma, then, may be described as a Megalodon-like 

 shell, the dental characters of which, however, are peculiar, com- 

 bined with the external figure oi Isocardia and Opis. 



Example. Pachyrisma grande. 



Testa cordata, elongata ; carina obtusa, dorsali, postica, latere antico brevi ; latere 

 postico profunde depresso ; striis numerosis, concentricis, irregularibus. 



