204 LAND REOIONS. 



Grastrodonta, Helicodiscns, Polygyra, Stenotrema, Triodopsis, 

 Mesodon, etc. Yet few as the Helices are, they outnumber the 

 Bulimi and Pupae, which are mostly minute, and also unicolored. 

 Succinea is well developed, and there are a few slugs. Only two 

 species of opercalates are native to the region.* On the other 

 hand the development of the fluviatile forms is enormous, and is 

 unequaled in any other region. The numerous large rivers and 

 lakes are not only well populated, but in many cases each 

 river system has forms restricted to itself. 



The so-called American Melanians belong to a group entirely 

 restricted to the United States, and form a distinct family, 

 Strepomatidse. The animal differs from the true or oriental 

 Melanians by the absence of a fringed border on the mantle and 

 by their oviparous reproduction. In 1875, I published a mono- 

 graph of the Strepomatidse, containing nearly 450 species after 

 making wholesale reductions on the number of described species. 

 Since the preparation of that monograph, a few additional forms 

 have been characterized. The family includes 9 generic groups : 

 lo 3 species, Pleurocera 83, Angitrema 12, Lithasia 13, Strepho- 

 basis 9, Eurycselon 10, Groniobasis 255, Schizostoma 28, 

 Anculosa 31. Most of these are Southern species, the Alabama 

 River and its tributaries, the Coosa and Black Warrior, being 

 the most thickly and variously populated. The waters of the 

 N. England, Middle and Northwestern States contain but few 

 species, yet those which occur have usually a much more 

 extended distribu.tion than the Southern species. The genus 

 lo is entirely confined to the Tennessee River and its tributary 

 streams, the genus Schizostoma to the Coosa River. Few of 

 these shells occur west of the Mississippi River. The develop- 

 ment of Naiades is equall^^ extraordinary with that of the Strepo- 

 matidae. Mr. Lea, who has for half a century devoted unceas- 

 ing attention to this very difficult family, admits 645 American 

 specie's, of which are XJnio 559, Margaritana 28, Anodonta 58 ; 

 most of them first characterized by himself. The distribution 

 of the Unionidfe does not differ much from that of the Melanians ; 

 where the latter are most plentiful, the former also abound. 



Limnseidse and Viviparidge are both largelj' developed ; the 

 species of the first family are particularly numerous, but so 

 polymorphous that it is very difficult to define the species. 

 The American Yiviparidae include the special tjqaes Tulotoma, 

 and Lioplax. The European Paludina is represented by P. 

 lineata^ a species which has been considered identical with its 



* The limits of this work forbid pursuing more in detail the geographical 

 distribution )f American terrestrial mollusks : a valuable paper upon this 

 subject by Mr. Wm. G. Binney, is contained in Bull. Mus. Oomp. 

 Zoology, iv, 17. 



