620 BULLETIN UNITED STATES GEOLOGICAL SURVEY. 



form is only a more than usually elongate oval, and the real affinities 

 of some of the quadrate forms are more nearly with the triangular than 

 with some others of a quadrate outline. Indeed, the massive triangular, 

 plicate, and nodulous shells are almost as distinctly North American in 

 type as are any of those which are indicated in the foregoing table. 

 However, the tabular groupings given in this paper exhibit somewhat 

 clearly the characteristics of the North American Unionidm, and espe- 

 cially the subordinate types of the genus Unio. 



By observing the characteristics which this great family presents in 

 different continents and in different parts of the same continent, one 

 becomes strongly impressed with the idea that local or circumscribed 

 influences, either near or remote, have variously modified its evolution. 

 For example, omitting present consideration of many of the genera of 

 the family, the living Unionidce of Europe comprises very few species, 

 all of which have a smooth or i)lain surface, and none have so massive 

 a test as many of the American species have, nor are any excessively 

 shortened, like the subrotund, quadrate, and other forms that are 

 found in other parts of the world. The Unionidce of the New England 

 States are, as a whole, almost a counterpart of those of Europe, while 

 the streams south of that region, which empty into the Atlantic and 

 Gulf of Mexico, contain quite a different set of species and types. 

 Again, the great Saint Lawrence Eiver system, including the great lakes, 

 has, compared with other North American rivers, a meager Unione 

 fauna, so far as specific representation is concerned, and the rivers 

 which empty into the Pacific Ocean have still fewer specific representa- 

 tives of this great family. The waters of the great Mississippi E,iver 

 system not only contain the richest Unione fauna in the world, but even 

 a casual examination will show that the fauna of this river system com- 

 prises nearly all the forms that have been so generally designated as 

 North American types. The smaller rivers of the Gulf drainage not trib- 

 utary to the Mississippi contain among their numerous species some of 

 those peculiar types also ; but few or none, other than the simpler forms, 

 are found in those waters of North America that lie to the west, north, 

 and northeast of the great Mississippi River sj'stem.* 



The discovery in different parts of the world of so many fossil species 

 of the Unionidce in strata of different geological periods, extending back 

 at least as far as the earlier Mesozoic epochs, and which ancient species 

 differ so little in type from forms now living, suggests as legitimate and 

 natural the inference that the living species are lineally descended from 

 the fossil ones, and that they were not evolved in recent times in the waters 

 that now form their habitat. We feel warranted in assuming that the 

 conditions of Unione life have been preserved unbroken, in some cases 

 at least, if not in others, notwithstanding the physical changes that 

 have taken place during the Mesozoic and Tertiary periods. 



* It is a significant fact that those North American rivers which contain the richest 

 Unione fanna drain Mesozoic and Tertiary regions, while those that drain only Paleo- 

 zoic and Azoic regions have a comparatively meagre Unione fauna. 



