234 UNITED STATES GEOLOGICAL SURVEY OF THE TERRITORIES. 



these characters with that genus as defined by Forbes, Nyst, IT. and A. 

 Adams, or any one else, but that, their hinges correspond in all essential 

 details with the group here under consideration, as the shells do in other 

 respects. It will also be noticed that, in giving a diagnosis of the genus 

 Poromya (not from the typical species, but evidently partly, at least, from 

 the Cretaceous species before him), Dr. Stoiiczka departs entirely from all 

 of the previous descriptions, of Forbes, Nyst, Deshayes, and others, of that 

 genus, and describes the hinge as having "a strong tooth in the left valve, and 

 two smaller ones in the right, 1 ' and says that in each there is " a posterior 

 groove, in which was lodged a small cartilage; " also, adding that "the pallial 

 line is submarginal, and the posterior sinus small."* The latter character, 

 however, he may have taken from the true Poromya, as he does not illus- 

 trate or mention the pallial line in connection with his figures or descriptions 

 of the Indian Cretaceous shells; though they and our types may, and prob- 

 ably do, possess a small pallial sinus. 



In regard to the little "posterior groove," supposed by him to have been 

 for the reception of a small internal cartilage, I feel quite confident, from 

 examining his figures, that this is only the pit, I have mentioned in the right 

 valve of our shells, for the reception of the rudimentary posterior cardinal 

 tooth of the left; which small tooth he overlooked in his generic diagnosis, 

 though he illustrates it in one of the figures of his /'. superba, and actually 

 mentions it in his specific description of that shell. That the pit men- 

 tioned by him, or any of those described by me, could have been for the 

 reception of an internal cartilage, I cannot for a moment believe, for the 

 simple reason that in our shells at least (which I am confident present no 

 generic differences from the Indian species mentioned), each one of these 

 pits exactly corresponds to, and is entirely filled by, a tooth, or, slight pro- 

 jection in the hinge of the opposite valve. 



I have been, I fear, tediously particular in defining this genus, and in 

 pointing out the characters in which it differs from Poromya proper, as well 

 as its identity with the Indian shells mentioned, not only because these Cre- 

 taceous shells have been wrongly confounded with Poromya, but also because 

 I have been rather sharply criticised by Dr. Stoiiczka for proposing the 

 groups Liopistha and Cymella. Tin; first he insists is not distinct from Phola- 

 domya, because, he thinks he has found Pholadomya caudata, Roemer, or at 



* Palaeont. Indica, III, 38. 



