202 BULLETIN OF THE 



From Lower California and Mexico the Pacific Region has been 

 •shown to be equally distinct, wanting entirely the Holospira, Glandina, 

 Bulimulus, and Zonites of those regions. 



Failing on the north, east, and south, the west alone is left fo us from 

 whence to trace the pulmonate fauna of the Pacific Region, and here 

 the secret of its origin lies buried under the Pacific Ocean.* 



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: — 



Patula strigosa. Patula Horni. 



Cooperi. Helix polygyrella. 

 Haydeni. 'Mullani. 



Idahoensis. Pupa Arizonensis. 

 Hemphilli. hordeacea. 



The first two of these species, perhaps identical, are also found on 

 the eastern slope of the Rocky Mountains, in Wyoming and Dakota. 



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

 culiar- to it, the following species from the Pacific Province, inhabiting 

 either slope of the Sierra Nevada : Vitrina Pfeifferi, Zonites Whit- 

 •neyi, Pvpa corpidenta, Succinea SiUimani, and Succinea Stretchiana. 

 The following, also, from the Oregonian Region of the Pacific Province, 

 Helix devia, Helix Toivnsendia?ia, and Macrocyclis Vancouverensis, are 

 found at its most northern point, though the former two species are re- 

 duced in size. We find, also, over the Central Province the following 

 species, whose derivation can readily be traced to the north ; f Zonites 



* A subsidence of eight hundred feet in tire continent of North America would 

 leave on its eastern shore a strip of land of about equal size of our Pacific Region, 

 equally distinct in its terrestrial mollusca from the balance of the continent. In 

 this case, however, we should have a distant island of the Appalachian chain on 

 which we should find all the species of the eastern coast of the mainland. This 

 would give us a proof of what we can now only suspect as regards the Pacific Prov- 

 ince, — of former more wide distribution of its pulmonate fauna. From wherever 

 the fauna may have originated, we can easily explain its present condition. The 

 physical and climatic features of the Pacific Region are such as readily to account 

 for its richness in terrestrial mollusks in comparison with the less favored Central 

 Province, and even with the Eastern Province. 



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

 p. 204. 



