A MANUAL OP AMEEICAN LAND SHELLS. 29 



continent several small species now found in Florida and Texas, and 

 even in Mexico and tlie West Indies. 



{b) The Interior Eegion lies to the south of the Northern Eegion, but 

 extends onl^'^ as far as the Rocky Mountains* on the west. Southerly 

 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 laud mollusks in former geological times. In the Post- 

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

 mense 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 

 center the species have extended over the region; some of them also 

 have passed the barrier of the Appalachian chain into the JS^orthern 

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

 into the Southern Region. Another theory might suggest that the Cum- 

 berland Subregion was the point of origin of all the species, those still 

 restricted to that subregion not being adapted to the wider distribu- 

 tion which the other species have obtained. Any one familiar with the 

 habits of snails is well aware how much they differ in this respect. 

 Some are much more disposed to migi-ate than others. Thus, Triodop- 

 sis appressa is content to remain within a radius of a few feet under a 

 decaying log; Mtsodon thyroides is more restless, travels much, and 

 climbs trees ; Tachea nemoralis has no local attachments, migrating far 

 and wide. These facts I have verified in my own garden during many 

 years. The Triodopsis appressa spoken of are descendants of Illinois 

 specimens given me twenty-five years ago by the lamented Kennicott. 



I will here mention that a colony of T. appressa has lately been found 

 in the island of Bermuda, no doubt introduced on plants. 



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

 rare, owing to the nature of the soil. For a description, see Terr. Moll. U. S., Vol. I, I. c. 



tSee Terr. Moll U. S., Vol. I, 185. It must be remembered that the glacial epoch 

 would not destroy this fauna, as the ice-sheet did not extend over the southern por- 

 tion of the region. Here the species would be preserved, and from hence, after the 

 disappearance of the ice, they would repeople the whole region, 



