24 TERRESTRIAL AIR-BREATHING MOLLUSKS. 



II. THE CENTRAL PROVINCE extends from Mexico to the British 

 Possessions, between the Rocky Mountains on the east, and the Sierra 

 Nevada and Cascade Mountains on the west. 



The following are the species peculiar to the province : 



Limax montanus. Patula Horni. 



Patula strigosa. Microphysa Ingersolli. 



Cooperi. Polygyrella polygyrella. 



Haydeni. Mesodon Mullani ( = devia). 



Idahoensis. Pupa Arizonensis. 



Hemphilli. hordeacea. 



The second and third of these species, perhaps identical, are also 

 found on the eastern slope of the Rocky Mountains, in Wyoming and 

 Dakota, in company with P. solitaria. I have shown above that the 

 last-named species has penetrated the Central Province, and even passed 

 the barriers of the Pacific Province at the Dalles. 



To the above must be added, as inhabiting the province, but not 

 peculiar to it, the following species from the Pacific Province, inhabiting 

 either slope of the Sierra Nevada : Vitrina Pfeifferi, Zonites Whitneyi, 

 Pupa corpulenta, Succinea Sillimani, and Succinea Stretchiana. The fol- 

 lowing, also, from the Oregonian Region of the Pacific Province, Mesodon 

 devia, Arionta Townsendiana, and Macrocydis Vancouver ensis, are found at 

 its most northern point, though the former two species are reduced in size. 



be found on the North American Continent, nor would any record be preserved of the 

 former connection of the regions. Indeed, no one would then suspect that the tropical 

 genera Glandina, Veronicella, and Cylindrella had ever been represented on the eastern 

 portion of this continent. 



The West India Islands being much more widely separated from North America, the 

 presence among them of the small American species (catalogued ou p. 37) would be still 

 more difficult to explain. 



A fain the supposed subsidence would destroy most of the species peculiar to the Sub- 

 Region of Texas (see p. 37), and remove the evidence of the present intermingling of the 

 North American and Mexican faunas in that Sub-Region. 



Another effect would be to remove from our reach all evidence of the origin of our 

 species in Post-pleioceue days, the fossil deposits in the bluffs being rendered inaccessible. 

 Thus one would not be able to have correct impressions of the origin and distribution of 

 certain species. The non-pulmonate Helicince give the best instance of this. Finding 

 Helicina orbiculata and occulta confined to the narrow limits of the Appalachian Island, 

 one would have no reason to suspect their past history has been so much more interesting 

 than that of many of the species of Stenotrema, etc., found with them, which never had 

 had a larger distribution. It would be impossible to know that Helicina orbiculata and 

 occulta flourished greatly in Post-pleiocene times ; that later, one of them, occulta, became 

 comparatively rare and restricted in range, while orbiculata became very numerous in 



