WALKER ON ORIGIN AND DISTRIBUTION OF MOLLUSCA. 49 



to Florida. In the Boreal Region some of these forms have a wide 

 range to west extending as far as Manitoba, if not further, and from 

 this region some of them have acquired a considerable range into the 

 states south of the great lakes. As a whole this fauna has closer rela- 

 tions with the Californian and Eurasian fauna than with the species 

 peculiar to the interior region of this continent. Thus the European 

 Margaritlna margaritifera is common to both the Pacific and Atlantic 

 states, but curiously enough is wholly wanting in the broad territory 

 lying between them. The Anodontas are also very similar to those of 

 California, but are sufficiently different to be generally accorded specific 

 distinction. The TInios while peculiar to the region have no relation to 

 the European forms, and as already stated, this genus does not occur in 

 the Californian Province at all. In the immense region comprising the 

 greater portion of the continent lying between these narrow coast 

 provinces is to be found an exuberance of Unione life, as is without 

 parallel in any other portion of the world. Here under the kindly in- 

 fluences of what must be a peculiarly favorable environment, are to be 

 found a multitude of species which in size, shape and manner of orna- 

 mentation exhibit almost infinite variety, and which nevertheless are 

 throughout stamped with such local peculiarities that to even the tyro 

 in conchology, no label is needed to indicate their fatherland. 



But even in this great assembly of similar, yet dissimilar forms, there 

 can without difficulty be distinguished two great races, or faunal groups. 

 The one, and by far the larger one embracing the massive triangular, 

 plicate and nodulous forms, which are distinctly North American types, 

 has its headquarters in great valley of the Mississippi and from thence 

 has spread out northerly into the St. Lawrence valley and southerly into 

 the rivers of Texas and Alabama. 



To the southeast, however, it is to a large extent replaced by a numer- 

 ous group of smaller and plainer species which, as though from a met- 

 ropolis in the mountains lying between Tennessee, Alabama, the Caro- 

 linas and George, has peopled the mountain streams on either side, 

 east toward the Atlantic and south and west to the Gulf with a multitude 

 of forms wdiose susceptibility to local influences has played almost as 

 much mischief with current standards of specific distinction as their 

 neighbors and associates, the Pleuroccridw from the same region. 



Besides the Uniondce, there is but one other family of bivalve mollusks 

 represented in our existing fauna. The Cyrenidce represent a large 

 number of species of small size (the largest being one-half inch and the 

 smallest less than one-fifteenth of an inch in its greatest diameter) of 

 general distribution. Some of them indeed ranging over nearly the 

 whole continent. Like the Linmwidw among the univalves, it reaches 

 its maximum development in the north, and from thence has apparently 

 extended southward in all directions. As a necessary consequence the 

 great mountain ranges, which have so effectively limited the range of 

 the Unionidw have apparently had no influences in determining the range 

 of these little species. 



Passing now from the consideration of the distribution of the various 

 orders and families represented in our fauna and collating the details 

 of their distribution in order to get a general idea of the leading features 

 of our fauna, as a whole we find that both among the land and fluviatile 

 species evidence, which is substantially the same in both classes, tending 

 7 



