WALKER ON ORIGIN AND DISTRIBUTION OF MOLLUSCA. 47 



the barriers, which seem to have been sufficient to determine the range 

 of the land species, have apparently been overcome or, at least, do not 

 apply. 



Thus the fresh water pulmonate families, while they have their metrop- 

 olis in the north, like the land species peculiar to the Boreal Region, 

 probably from greater powers of endurance and greater adaptability 

 to environmental changes, and possibly also from the greater facilities 

 for migration afforded by the medium in which they live, have a wide 

 distribution. Thus while the region of the Great Lakes and the St. 

 Lawrence drainage system has the largest collection of species, yet there 

 is not a state in the Union that has not some representatives of nearly 

 every genus, and some species can be found in nearly every state. In- 

 deed these genera have spread over the whole world, and some species 

 are not only circumpolor, but almost cosmopolitan in their range. 



These forms have been, almost everywhere in this country, also ac- 

 companied by certain genera of small operculate mollusks, such as 

 Valvata and Amnicola, etc. As has already been stated in regard to the 

 distribution of the boreal land species and for similar reasons, the present 

 range of these forms was undoubtedly effected from the north to the 

 south. 



In the operculate family of the Pleuroccridce, or American melanians 

 we have, as in the Polygrw among the land snails, a group of wholly 

 American origin and one surprisingly like that in its distribution. Of 

 enormous abundance (Tryon's monograph, 1874, containing 464 species) 

 in the prolific rivers draining the lower parts of the Appalachian chain, 

 where it exists in such an infinite variety of forms as to almost do away 

 with any attempt to define specific limitation, it has spread out in all 

 directions. The great majority of these species are confined to the rivers 

 flowing from the Cumberland mountains and in these they are generally 

 confined to the upper portions as they are particularly partial to rapidly 

 fiowing streams with rocky bottoms. On the west the Mississippi river 

 seems to have been a barrier and but few species are found in the states 

 lying west of it. Curiously enough a colony of detached species of peculiar 

 aspect is found in the Californian Region, a circumstance analogous to 

 the group of Californian Polygyrw already mentioned. A remarkable 

 genus of this family, ^chizostoma, embracing nearly thirty species, is 

 confined to the Coosa river in Alabama. To the north but few species 

 have extended beyond the Ohio ; a very few reach into the rivers tributary 

 to the great lakes — thirteen species being found in Michigan. From Vii-- 

 ginia to the north, the Appalachian range has proved an effective barrier 

 against immigration into the Atlantic states. New England has no 

 representative of the family. A single species of Goniohasis, which singu- 

 larly enough is not found west of these mountains, and a species or two 

 of Anculofia are the only representatives in eastern New York and 

 Pennsylvania. 



The remaining family of operculate mollusks represented in our 

 fauna, the Viviparidw, which includes the larger forms usually met 

 with, is also peculiar in its distribution. The typical Yiviparw have a 

 widespread range in the Northern Hemisphere, being well represented 

 in Europe, and exceedingly abundant in Southern and Eastern Asia. A 

 curious example of local development may be mentioned in connection 

 with this genus. The species are many of them ornamental with trans- 



