780 PROCEEDINGS OF THE NATIONAL MUSEUM. voL.xxnr 



Most of the older names have been treated with indifference, and the 

 same group has several times received a succession of names from 

 authors who did not investig-ate the history or literature relating to it. 



^Vhile it would be too much to expect that the present revision is 

 al)solutely free from error, it is believed that it takes a considerable 

 step in advance over anything now published, and will at least direct 

 attention to a very interesting group of Pelecypods. 



The families included in the present revision are as follows: 



THYASIRID.E. 



Both coasts; 35 species in all. East coast, 28: west coast, 11. 



DIPLODONTID^. 



Both coasts; 20 species. East coast, 13; west coast, 9. 



LUCINID.E. 

 Both coasts; 63 species. 



CORBID^. 



Exotic? (Eastern Tertiaries.) 



CYRENELLID^E. 

 East coast; 2 species. 



All of these except the penultimate are represented in our recent 

 fauna, and all in our Tertiary fauna. There are in all 120 species, of 

 which 81 belong to the Atlantic, 45 to the Pacific, and only 5 (or possi- 

 bly 6) are common to the two sides of the continent. The relative 

 richness of the Atlantic coast is very marked, but of the Pacific species 

 a large proportion, though not actually identical, are at least closely 

 representative of Atlantic species, and doubtless are derived from a 

 not remote common ancestor. As regards the Tertiary species, it may 

 be said that, while nearly every recent group or species has its fossil 

 analogue, we find as we recede in time, especially in the Eocene, a ten- 

 dency for the subdivisions to coalesce or at least to lose their distinc- 

 tive features and exhibit a mutability of character which, from the law 

 of evolution, is exactly what we ought to expect. Contrary to my own 

 anticipations, the superficial and ornamental characters are those which 

 appear to be most strongly conserved from one horizon to another, 

 through a series of geological epochs. Such features frequently come 

 down from the Cretaceous or Lower Eocene with practically no change. 



After satisfying myself that there was no mistake in this generaliza- 

 tion, 1 concluded that this might be accounted for on the hypothesis 

 that these characters, mostly due to trifiing nmtations of tiie armature 

 of the mantle edge, are so little connected with essinitials in the lives 

 of these animals that, having Ix^en once accjuired. natural selection has 

 little or no influence upon them, and therefore rarely sets up anv tend- 

 ency to change. 



