THE NAUTILUS. 



VOL. XXV. JUNE, 1911. No. 2 



THE DISTRIBUTION OF OAHTJAN SPECIES OF AMASTRA. 



BT HENRY A. PILSBRY. 



The genus Amastra, comprising dull ground-snails of Achatinellid 

 type, is found throughout the Hawaiian group from Kauai to Hawaii, 

 but is rather sparingly developed on the two terminal islands, reach- 

 ing its maximum in the intermediate islands, where special groups 

 have been differentiated. In this intermediate region there have 

 been two main centers of evolution, the one composed of Maui, 

 Molokai and Lanai, the other of the single island Oahu. Common 

 to these two centers are the group Cyclamastra (represented by 

 small umbilicate species, like A. umbih'cata, now mostly extinct), 

 and its derivative Pterodiscus. 



The two centers are roughly comparable topographically. Oahu 

 consists of two mountain ranges, a western (Waianae range) and a 

 much longer northeastern or Main Range, separated by a lower 

 tract, not forested, and supporting no Amastrae. In the other center, 

 Molokai -f Maui represent the Main Range of Oahu, Lanai the 

 Waianae range; only there has been subsidence isolating the com- 

 ponent ranges. This may seem an idle analogy; but it was sug- 

 gested by the facts of molluscan distribution. Lanai by its Amastrae 

 holds such a relation to Molokai as the Waianae range to the west- 

 ern end of the main range; while Molokai, west Maui and east Maui 

 are almost as closely related in their Amastraa as corresponding 

 segments of the Main Range of Oahu, if this was broken by dropping 

 out a few valleys in two places. 



Omitting a few species of which the exact locality is unknown, and 



