216 UNITED STATES GEOLOGICAL SUKVEY OF TIIE TERRITORIES. 



that I should not have suspected it to be the same, or even a nearly allied 



* 



species. If Sowerby's species varies, however, so greatly as to include the 

 forms figured by Professor Agassiz under the same name, and placed by him 

 in his group Bucardince, the latter group would be at least partly identical 

 with Procardia, and thus show such a transition from that type to the section 

 Bucardince, that we might perhaps properly range tbe latter as a section of the 

 subgenus Procardia. If, on the other hand, this transition between Sowerby's 

 type and that figured by Professor Agassiz under the same name, has not 

 been more decidedly established than can be done from the published figures, 

 I should think Professor Agassiz's groups Bucardince and Cardissoides would 

 properly form one, or perhaps several, distinct subgenera from Procardia. 

 His names, however, are unfortunately not such as could be used for sub- 

 genera. 



The genus Pholadomya is closely related to Goniomya, Ccromya, 

 Myacites, Pleuromya, Clnmomya, Allorisma, &c, but differs from them all 

 in constantly having radiating costse. In Ceromya, it is true, there arc rarely 

 faint radiating costfe ; but in that group the beaks are more generally sub- 

 spiral, and the general physiognomy different, while on the different laminae 

 the surface-markings of these shells (which are thicker than in Pholadomya} 

 are not parallel, but run in various directions. Goaiomya is also costate, but 

 the costse have a curious divaricating arrangement, and never radiate from 

 the beaks ; while the shell likewise differs in usually, if not always, having 

 a very minutely-granulated surface. Chanomya, in addition to its non- 

 costate and granular surface, also differs in its much more widely gaping 

 posterior extremity. 



Although species have been erroneously referred to the genus Phola- 

 domya from older rocks, it seems highly probable that it commenced its 

 existence during the Liassic period. The species are more numerous, how- 

 ever, in the later members of the Jurassic system, during the deposition of 

 which it attained its greatest development. It, was also well represented 

 through the Cretaceous period, but much more sparingly in the Tertiary; 

 and but one species (P. Candida, the type of the genus) is known to be living 

 at this time. This occurs in the West Indian seas, being found after storms 

 on the shores of the island of Tortola, thrown up, as supposed, by the waves 

 from deep water. 



