206 BULLETIN OF THE 



The Northern Region does not differ in the characteristics of its fauna 

 from that lying south of it, but its climate is too severe for any but the 

 more hardy forms. Thus, we find only the small Zonites and Helix, 

 with the genus Vitrina. Compared with the balance of North America, 

 the Region is peculiar for the great distribution of its species east and 

 west, owing to the mountain-ranges having here lost the great elevation 

 which they have farther south, and thus ceasing to be barriers to distri- 

 bution. The Region is also interesting as being the source from whence 

 have spread southward over the whole continent seyeral small species 

 now found in Florida and Texas, and even in Mexico and the West 

 Indies. 



(b.) The Interior Region lies to the south of the Northern Region, 

 but extends only as far as the Rocky Mountains * on the west. South- 

 erly it extends to the alluvial regions of the Atlantic and Gulf coasts, 

 the dividing line here not being sharply defined. 



This is the only portion of the continent where we have evidence of 

 the origin of our land mollusks in former geological times. In the 

 Post-pleiocene deposits along the Ohio and Mississippi Rivers are found 

 immense beds of shells, " proving that our existing species were living at 

 a period which, though recent in a geological sense, was anterior to the 

 last geological revolution, when the surface of this portion of the earth 

 was brought to its present condition, and to the existence of the higher 

 order of animals which now inhabit it, and even to that of the extinct 

 mammalians which are known only by their gigantic remains." t 



From the evidence gathered from these deposits, it appears that the 

 fauna of this Region can be traced to Indiana and Ohio. From this 

 centre the species have extended over the Region ; some of them also 

 have passed the barrier of the Appalachian chain into the Northern 

 Region, and some have spread, with the enlargement of the continent, 

 into the Southern Region. 



The following species have actually been found fossil in the Post- 

 pleiocene deposits: — f 



Zonites arbor eus. Zonites intertextus. 



fuliginosus. ligerus. 



inornatus. gularis. 



* This is the extreme limit, but before reaching it the land shells have become 

 very rare, owing to the nature of the soil. For a description see A. Binney, 1. c. 

 t A. Binney, Terr. Moll. U. S., I. 185. 



