34 TERRESTRIAL AIR-BREATHING MOLLUSKS. 



having, no doubt, crept along the mountain chain ; but no other of the 

 species of the Cumberland Sub-Region has been found as far north, 

 excepting Z. demissus. This last-named species is. found in a highly 

 developed state in Eastern Tennessee, and has extended into Western 

 Pennsylvania, North Carolina, Georgia, Alabama (near Mobile), and 

 Arkansas in a much dwarfed condition. 



If to the twenty-nine species catalogued above as peculiar to the 

 Sub-Region are added the sixty-six species which inhabit it as a portion 

 of the Interior Region (see p. 32), it will be seen that in the Cumber- 

 land Sub-Region we find the largest number of species of any portion 

 of North America. The Sub-Region is equally prolific in individuals, 

 and the individuals are highly developed. These facts are partially 

 explained by the nature of the country. Low mountains, thickly 

 shaded, well-watered, and with a genial climate and proper soil, offer in 

 their thickets and ravines innumerable safe breeding-grounds for the 

 land shells. 1 There seem also to be in this Sub-Region conditions 

 peculiarly conducive to testaceous variation. Six (or twenty per cent) 

 of its peculiar species are cai'inated, and here also the following spe- 

 cies of the Interior Region show the same tendency to carination, — 

 Zonites ligerus, intertextus, Patula alternata, Triodopsis appressa and 

 pcdliata. Here, also, we first notice the variation of Patula alternata 

 towards heavy ribs upon its shell ; which is still more apparent as the 

 species extends towards the southwest. 2 Here, also, Mesodon elevata is 

 often found banded. 



The Cumberland Sub-Region is peculiar for the development of 

 Zonites, and in the disintegrated genus Helix for the development of 

 the section or genus Stenotrema, almost peculiar to these narrow limits. 



(c.) The Southern Region comprises the peninsula of Florida, with 

 the adjacent islands, together with the alluvial regions of the Atlantic 

 and Gulf coasts. It includes, therefore, the eastern portion of North 

 Carolina, South Carolina, Georgia, all of Florida, the southern part of 

 Alabama, Mississippi, Louisiana, extending into Texas. 3 Its boundaries, 

 however, are but imperfectly known, and probably not accurately de- 

 fined. Many of the species from the Interior Region and Cumberland 



1 See Vol. I. pp. 122, 123. Being less adapted for cultivation than the balance of East- 

 ern North America, we may hope for the preservation of our land shells in this Region, 

 while they decrease rapidly before the advance of civilization elsewhere. See Ibid., pp. 

 132. 133. 



2 This heavily ribbed form was common in Postpleiocene days. 

 8 See Vol. I. 120, for a description of the Region. 



