THK NAUTILUS. 113 



of animals is generally toward the setting sun, it would be more 

 reasonable and more in harmony with this general law to base the 

 distribution of animal life on a westward movement across each con- 

 tinent, spreading north and south as food and climatic conditions 

 were found to be favorable to the existence of each class of creatures, 

 rather than upon a haphazard exodus of animals from Asia via 

 Bering Strait bridge. 



Undoubtedly a few shells have been introduced into America from 

 other continents, but, after two centuries of close commercial inter- 

 course between America and Europe, we can count all the known 

 introduced land shells on the fingers. I venture to suggest that the 

 distribution of animal life is determined by the laws of attraction and 

 repulsion as much as the revolutions of the earth in its orbit around 

 the sun. There are life centres on each continent around which 

 animals revolve, and from which they radiate and to which they re- 

 turn, with possibly a westward tendency of these life centres. In 

 obedience to this law of attraction birds return each spring to their 

 old nesting places ; some fish, like the salmon, return each season to 

 the rivers and creeks in which they were hatched to deposit their 

 spawn, and many other circumstances of a similar kind might be 

 cited in support of such a theory. 



I have visited all of the islands off the coast of Southern Cali- 

 fornia, except San Miguel and Anacapa, for the purpose of collecting 

 shells, but before presenting a complete list of the land shells. I will 

 offer descriptions of some forms that seem to be undescribed. 



In referring to the Helices I use the general term " Helix," under 

 which genus they have been described, and which, it seems to me, is 

 quite as suggestive, and certainly as useful, as the long cumbersome 

 names that have been recently adopted ; leaving to others the choice 

 of half a dozen or more genera and subgenera to which they have 

 been referred from time to time by several distinguished eastern and 

 foreign conchologists. 



[7b be concluded.^ 



A NEW AMNICOLA. 



MV BRYANT WALKER. 



Amnfcola letsoni. 



Shell small, elevated, solid, thick, white ; subimperforate, whorls 



