THE MOLLUSCA 959 



Most of the families have representatives in all portions of the 

 country where suitable conditions of environment are to be found. 

 But there are some notable exceptions. Thus the Viviparidae, 

 which form one of the most conspicuous elements of the fauna of 

 the Eastern States, are not found west of the Mississippi Valley. 

 The AmpuUaridae, which replace the Viviparidae in the tropics, are 

 only found in Florida and Southern Georgia; while the great 

 family of the Pleuroceratidae, the one family peculiar to our fauna, 

 is not found west of the Mississippi Valley drainage, except for a 

 few isolated species that occur in the northern Pacific states. 



Many of the genera have a general distribution in all parts of 

 the continent. That is, some representatives of such genera will 

 be found practically every place where suitable conditions obtain. 

 But it is to be borne in mind that comparatively few species have 

 a general distribution. Many of them have, so far as known, a 

 very limited habitat, while others range over a large extent of 

 territory. Many of the genera are likewise restricted to certain 

 portions of the continent and particularly to certain drainage 

 systems, while others are confined to a very limited area. The 

 Coosa River in Alabama, in this respect, has a most remarkable 

 fauna. No less than six genera and very many species are known 

 to occur only in it and its tributaries. 



But while, in a general way, it is true that our fauna as a whole 

 is well known, and its distinctive characters recognized, yet the 

 sum total of our present knowledge, large as it may seem, is very 

 small and inadequate when compared with what we might and 

 would like to know about it. 



But a very small portion of the continent has been collected over 

 with any sort of thoroughness, and there are undoubtedly very 

 many new types, both generic and specific, yet to be discovered. 

 While, of course, the mere description of new species is the least 

 important part of the work of the conchologist, yet the accurate 

 knowledge of all the species found within our borders is the basis 

 for the solution of the larger problems of distribution and evolution, 

 which are of great importance. The exact range of very few, if 

 any, even of our most common species is exactly known. It is 

 very desirable that lists of the species occurring in all the states 



