618 BULLETIJf UNITED STATES GEOLOGICAL SURVEY. 



Genera of tlie family Unionidce. 



A glance at the foregoing table will show that of the three genera 

 represented in North America two of them, Unio and Anodonta, are also 

 represented in every other part of the world, while the other^ Mar- 

 garitana, is more widely distributed than any of the remaining genera. 

 These three genera may therefore be properly regarded as the standard 

 or general types of the family; and as such they show its integrity in 

 this part of the world in a very conspicuous manner. This integrity is 

 still further conspicuously shown by the fact that in all parts of the world 

 the species of all the other genera Q's.c&^i Monocondylea (which has twenty- 

 six species) are very few, ranging only from one to nine species to a 

 genus, and still farther shown by the fact that the genus Unio embraces 

 fully four-fifths of all the species that belong to the family. 



Dr. Isaac Lea assigns only a subgeneric place to TJnio; but other 

 authors have given it a higher than generic rank by dividing it into 

 several sections which they raise to the rank of genera. Most of these 

 subdivisions of Unio are based upon the structure of the. soft parts 

 alone, while the shell in all cases possesses thfe distinguishing character- 

 istics of Unio. Such a basis for the subdivision of this great genus is 

 therefore beyond the cognizance of the paleontologist; but, nevertheless, 

 very definite lines of differentiation within its broad limits are indicated 

 by the form, particularly the marginal outline and the surface-features 

 or ornamentation of the shell, and which characterize more or less 

 clearly defined sections of the genus. While a large proportion of such 

 types, or sections, of the genus U'nio as are thus indicated are found to 

 be common to all parts of the world, there are certain other types that 

 are either peculiar to, or are more than usually prevalent in, some one 

 continent or region. Thus, certain sections of the genus as above indi- 

 cated are found to be peculiar to our continent, and have therefore come 

 to be known as " North American types" of Unio. So characteristic of 

 North American waters are some of these subordinate divisions or types 

 that the exclusively African genera Spatlia, Iridinia, and ^theria, and 

 the likewise exclusively South American genera Plagiodon, Columha, 

 Mulleria, etc., are scarcely more indicative of their respective continental 

 generation. 



