GEOGRAPHICAL DISTRIBUTION. 23 



other hand, the only form which has any development in the Central 

 Province, Patula, is scarcely known in the Pacific Province. 



Compared with Eastern North America, or the Eastern Province, as 

 it is designated below, the Pacific Province is remarkable for the absence 

 of all the larger Zonites. The presence of the smaller species, also, may 

 perhaps be accounted for by migration from the north, so that the genus 

 Zonites cannot be considered as characteristic of the Province. The 

 genus Pupa is less common. The genera Tebennophorus and Pallifera, 

 so universally distributed in Eastern North America, are unknown, and 

 so are the southern genera Glandina and Bulimulust. On the other 

 hand, we find the genus Macrocyclis much more developed, and meet 

 several genera unknown in the Eastern Province, such as Ariolimax, 

 Binneya, Prophysaon, and Hemphillia. The genera of disintegrated 

 Helix are proportionally more developed in the Pacific Region, and are 

 represented by quite dissimilar subgenera. The genera so peculiar to 

 the Eastern Province, Polygyra, Stenotrema, Triodopsis, Mesodon, are 

 scarcely represented. In their place we find Aglaia and Arionta, forms 

 unknown in the Eastern Province. The latter, though feebly repre- 

 sented in Europe, is characteristic of California. It is prolific of species 

 and also of varieties to a degree which has caused some confusion in the 

 synonymy. Glyptostoma is also peculiar to California. 



From Lower California and Mexico the Pacific Region has been shown 

 to be equally distinct, wanting entirely the Holospira, Glandina, Buli- 

 mulus, and Zonites of those regions. 



Failing on the north, east, and south, the west alone is left to 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. 1 



f * A subsidence of eight hundred feet in the 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 Province, 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 ill comparison with the less 

 favored Central Province, and even with the Eastern Province. In the supposed subsi- 

 dence in the Southern Region the change would be still greater. All the species peculiar 

 to it, catalogued on p. 35, would perish, excepting Jjitlimulus dealbatus. This species 

 would still be found in Kentucky, restricted to a small area ; all record of its former wide 

 distribution being at the same time destroyed. 

 The West Indian and South American species, catalogued on pp. 36, 37, would no longer 



