III. DISTRIBUTION IN SPACE. 



A. THE PRESENT DISTRIBUTION. 



The family Lymnaeidae is widely and generally distributed over 

 the North American continent from the Arctic Ocean to the Isthmus 

 of Panama. In almost every body of water, large or small, some mem- 

 ber of this family is almost sure to be found. In studying the geo- 

 graphical distribution of the family Lymnaeidae, the fact of the wide 

 dispersal of many of the species is notably apparent. Thus, palustris, 

 obrussa, stagnalis appressa, caperata and humilis modicella are of al- 

 most universal distribution and many of the other species, as catasco- 

 pium, parva, apicina, binneyi, etc., cover a wide range of territory. 



The evolution of the Lymnaeas has been very slow compared with 

 that of the vertebrates, or even with the land snails. The geologic 

 study of the family shows it to be of great antiquity, undoubted species 

 of Lymnaea having been found in the rocks of Lower Cretaceous age. 

 As these Cretaceous Lymnaeas do not differ greatly from the existing 

 species, it is evident that the ancestors of the genus must be looked for 

 in rocks of much earlier date, possibly Jurassic, or even an earlier 

 formation. 



In the distribution of this family we must, it seems evident, con- 

 sider these early Cretaceous species as the precursors of the present 

 Lymnseid fauna. When we study the distribution of the early Paleo- 

 zoic rocks we are at once struck with the fact that from no other 

 American source could the fauna have originated. 1 The larger part of 

 the Paleozoic rocks are of marine origin, and cover the greater part 

 of the American continent east of the Rocky Mountains. Mr. C. A. 

 White makes the following statement concerning the condition of the 

 continent when the non-marine fauna first appeared. 2 "East of west 

 longitude 95 (the western part of the Mississippi Valley), North 

 America is mainly occupied by Paleozoic and Archaean rocks, as is 

 also a large area which extends northward and southward through 

 western North America, the eastern border of which is not far from 

 the 113th meridian of west longitude. These two great areas are 



Merrill (Amer. Journ. Sci., iii, V, p. 467) asserts that the land and fresh- 

 water fauna originated in America. 



2 See Walker, Report Mich. Acad. Sci., p. 52, 1900, and also the various 

 papers of Mr. White, listed in the bibliography appended to the chapter on 

 Distribution in Time. 



