GEOGRAPHICAL DISTRIBUTION. 25 



"We find, also, over the Central Province the following species, whose 

 derivation can readily be traced to the north j 1 Zonites minusculus, ful- 

 vus, indentatus, Vallonia pulchella, Helicodiscus lineatus, Patula striatella, 

 Ferussacia subcylindrica. 



Arionta Rowelli, a Lower California species, is omitted from the list, 

 its presence in Arizona not being well authenticated. 



The fauna of the Central Province is quite distinct from that of the 

 Pacific Province, but is nearly allied to that of the Eastern Province, 

 its genera being the same, excepting Polygyrella. It may therefore be 

 of the same origin as the fauna of the Eastern Province. 



The paucity of species over this large province is owing to the nature 

 of its climate and soil, — causes in equal force on the western border of 

 the Eastern Province. 



In order to avoid mistakes in the study of the geographical distribution 

 of North American Land Shells, one must constantly bear in mind the 



individuals over a vast extent of territory ; and finally, that our supposed subsidence 

 gradually restricted them to the Appalachian Island. 



This supposition of subsidence might be carried still further, till we should have in cer- 

 tain islands of the Appalachian chain the sole resting-places of the now widely distributed 

 Eastern North American fauna. The more southern of these islands would alone retain 

 the species of the present Cumberland Sub-Region, and thus be much richer in species 

 than the more northern islands. On the other hand, these more northern islands would 

 possess species derived from the present northern regions which would not be found in 

 the southern islands. 



Still more instructive is the supposition of a subsidence in Eastern North America 

 which would leave above the level of the sea only two groups of islands, formed by the 

 White Mountains of New Hampshire, and Mount Mitchell and Black Mountain of North 

 Carolina. On the latter we may suppose would be preserved all the species given in 

 the lists on pp. 32, 33. Of these species all would be peculiar to the island, except 

 such as are named in the list on p. 30, which would all be found also in the White 

 Mountains, where we should also find the following species peculiar to the islands, 

 Mesodon Sayii, dentifera; Vitrina limpida ; Zonites milium, Binneyanvs, ferreus, 

 exiguus, multidentatus ; Patula striatella, asteriscus; Pupa decora; Vertigo Gouldi, 

 Bollesiana, simplex; Succinea Totteniana. Of the former distribution of these species 

 nothing could be known, but a former connection of the two groups of islands would be 

 surely indicated by the presence of so large a proportion of species common to each. A 

 former connection of the two groups of islands with Europe and Asia would be as surely 

 indicated by the presence on each of Zonites fulrus, nitidus, viridulus; Acanthinula 

 harpa; Vallonia pulchclii ; Ferussacia subcylindrica, and Pujia muscorum. Nor could 

 it escape the attention of conchologists that these and other small species, Z. arboreus, 

 etc. (see p. 27, note), proved that a former connection must have existed between these 

 groups of islands and the far-off Central and Pacific Provinces. 



1 See remarks on the distribution of these species over Eastern North America, 

 below. 



