GEOGRAPHICAL DISTRIBUTION. 29 



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." 1 



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. Another theory might suggest that the 

 Cumberland Sub-Region was the point of origin of all the species, those 

 still restricted to that sub-region not being adapted to the wider distri- 

 bution 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 migrate than others. Thus, Triodopsis 

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

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

 climbs ti'ees ; 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 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. 



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

 pleiocene deposits : - 



Zonites arboreus. Macrocyclis concava. 



fuligiiiosus. Patula solitaria. 

 inornatus. alternata. 



intertextus. perspectiva. 



ligerus. Helicodiscus lineatus. 



gularis. Strobila labyrinthica. 



1 See 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 portion of the Region. Hi-re 

 the species would be preserved, and from hence, after the disappearance of the ice, they 

 would repeople the whole Region. 



