40 TERRESTRIAL AIR-BREATHING MOLLUSKS. 



the Mesodon, Triodopsis, Stenotrema, etc., were already established in 

 post-pleiocene days. It is impossible to learn how much earlier they 

 appeared, but of one significant fact we are certain, — .they are more 

 recent than the elevation of the Rocky Mountains and Sierra Nevada, 

 for otherwise these chains would not form, as now, dividing lines between 

 the eastern, central, and pacific fauna. There are, indeed, several small 

 species which have passed these barriers, being found over all of North 

 America. These same species are found equally distributed in Asia and 

 Europe. They are undoubtedly of much earlier origin than the strictly 

 American species, and belong to some extinct fauna of world-wide dis- 

 tribution. The circumpolar connection of the three continents has fa- 

 cilitated their distribution. In this connection it is worthy of note 

 that one of our existing species, now confined to America (Strobila laby- 

 rinthica), is said to have existed in France in Tertiary days. 



Our Southern Region has evidently been peopled from other fauna 

 than that which supplied the Mesodon, Triodopsis, Stenotrema, etc., 

 of the Interior Region. It was, no doubt, from some now extinct semi- 

 tropical fauna that these came, but long enough ago to allow the Poly- 

 gyras, Glandinas, etc. to be modified into species distinct from those 

 which from the same common origin have become the equally well- 

 established West Indian, Central American, and Mexican species. 



The Central Province has, from geological causes, been more recently 

 peopled by pulmonata than the Eastern Province. Its local species are 

 less numerous. Patula is its characteristic genus, with species so vary- 

 ing and intermingling one with .the other that the student cannot 

 refrain from noticing that they have the appearance of a species in a 

 slightly advanced stage of evolution, each form not as yet established 

 as distinct, easily recognized species. 



The Pacific Province, also, presents in its variable, scarcely distin- 

 guishable Ariontas, a fauna of comparatively recent growth, but whence 

 its origin it is difficult to say. 1 



Finally, we have in the list of American land shells several species, 

 purely local in their distribution, imported through the more or less 

 direct agency of man. Of these, Pomatia aspersa was no doubt intro- 

 duced as an article of food by foreign residents of Charleston, S. C, and 

 seems to have established a hold there. 2 Zonites cellarius was intro- 



1 See Dr. Cooper, as referred to on p. 18. 



'• I have been asked what authority I have for this opinion, so think it worthy of state- 

 ment that Charleston specimens belonging to the cabinet of the late General Totten still 



