WHITE ON PALEONTOLOGY. 617 



and living ]S"ortli American forms that have been referred to that genus ; 

 and they have mnch the general aspect and characteristics of the Old 

 TYorld Melaniaus. They seem to constitute almost the only known foreign 

 element among our fresh-water types, either fossil or living. The two 

 species of Carimfex from the Tertiary strata of ISTevada represent the 

 only genus that is known to exist in the waters of the Pacific drainage, 

 and not in any of those which flow into either the Atlantic Ocean or the 

 Gulf of Mexico. This fact possesses peculiar interest, because closely 

 related representatives of nearly all the fossil land and fresh- water Mol- 

 lusks of Western ISTorth America are found now living upon the drainage- 

 slopes of the Alantic and Gulf of Mexico, while comparatively few of 

 them are represented among the living forms of the land and fresh 

 waters of the Pacific drainage-slope. 



Although the peculiar " Xorth American " characteristics before re- 

 ferred to are so plainly observable in nearly all the fresh- water and land 

 Mollusks of Western l!^orth America, it is the Unionidw that are found to 

 possess those characteristics most completely and invariably. Before 

 proceeding to a comparison of the fossil with the living forms of this 

 family, however, a brief comparison of the living Unionidce of the differ- 

 ent parts of the world will be necessary and proper. 



This family has been so variously divided and subdivided into genera 

 and subgenera by different authors that it is somewhat difficult to adopt 

 a classification that is free from more or less serious objections. The clas- 

 sification here used is, therefore, only provisionally adopted, and, in its 

 use, reference is had to the characteristics of the shell alone, for the very 

 obvious reason that these features only can be considered in the case of 

 fossil forms, for the discussion of which the classification here presented 

 is employed. It cannot, of course, be understood that these generic 

 names represent generic distinctions of equal value, for the fact is far 

 otherwise ; but they are presented and tabulated here to show more 

 clearly by contrast the generic differentiation that obtains in this fam- 

 ily in the different great divisions of the earth. Notwithstanding the 

 fact that there is a much greater profusion of Unione life in the fluviatile 

 and lacustrine waters of North America than in those of any other part 

 of the world, the existence of only three generic forms is recognized in 

 this discussion. This does not, however, imply a lack of diversity, for 

 these genera (especially the genus Unio) embrace a greater number of 

 species than are believed to exist on any other continent, not excepting 

 that of South America, where the generic forms are more than three 

 times as numerous as they are on our own continent. 



The following is a tabular view of the genera of this family as it is 

 represented upon the different continents respectively. 



