Ill. DISTRIBUTION INCSPACE: 
A. THE PRESENT DISTRIBUTION. 
The family Lymnzidz 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 Lymnezidz, the fact of the wide 
dispersal of mariy 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 specie$, as catasco- 
pium, parva, apicina, binneyi, etc., cover a wide range of territory. 
The evolution of the Lymnzas 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 Lymnza having been found in the rocks of Lower Cretaceous age. 
As these Cretaceous Lymnzas 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 
Lymneid 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. 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.? “East of west 
longitude 95° (the western part of the Mississippi Valley), North 
America is mainly occupied by Paleozoic and Archzan 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 
1Verrill (Amer. Journ, Sci., iii, V, p. 467) asserts that the land and fresh- 
water fauna originated in America. 
2See 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. 
