XI THE NEARCTIC REGION 339 



at one time undoubtedly existed between India and Eastern 

 Africa began to be less continuous,^ the Mascarene group was 

 first severed from what ultimately became Madagascar, while the 

 Seychelles, and perhaps the Comoros, still continued united to it. 

 The Comoros, which lack the great Helices^ separated off from 

 Madagascar first, while the Seychelles continued in more or less 

 direct union with that island sufficiently long to receive the pro- 

 genitors of Stylodonta (a peculiar group of Helix) ^ but became 

 disunited at an exceedingly remote period. 



E. The Nearctic Region 



The southern boundary of this region may be regarded as 

 roughly corresponding to that of the United States, ix. Lower 

 California and Mexico are excluded. The southern portion of 

 Florida belongs to the Antillean sub-region. 



The principal characteristic of the Nearctic Region is the 

 remarkable poverty of its land Mollusca. No district in the 

 world of equal extent is so poor in genera, while those which 

 occur are generally of small size, with scarcely anything re- 

 markable either in colouring or form. The elongated land shells 

 (^Clausilia^ Buliminus)^ so characteristic of Europe, are entirely 

 wanting, but a few Bulimulus^ of Neotropical origin, penetrate 

 Texas, and from the same sources come a few species of Grlandina 

 (as far north as S. Carolina), Holospira (Texas), and Helicina. 



The region falls into two well-marked sub-regions, the N. 

 American and the Californian, with the Rocky Mountain district 

 as a sort of debatable ground between them. The Californian 

 sub-region consists of the narrow strip of country between the 

 Sierra Nevada, the Cascade Mountains and the coast-line, from 

 San Diego to Alaska; the N. American sub-region consists of 

 the remainder of the region. 



(1) The N. American Sub-region. — The Carnivorous genera 

 are represented solely by the few G-landina mentioned above, 

 and by the indigenous genus Selenites^ a form midway between 

 Testacella and Limax, whose metropolis is on the Pacific slope, 



1 It is by no means implied that unbroken land communication between 

 India and Madagascar, across the Indian Ocean, ever existed. A series of great 

 islands, whose remains are attested by theChagosand other banks, would be quite 

 sufficient to account for the results, as we find them. See especially Medlicott and 

 Blanford, Geology of India, vol. i. p. Ixviii. 



