XVI INTRODUCTION. 



Laminella arose in and radiated from the Molokai-Maui cen- 

 ter, etcetera. 



V. 



Structural and Geographic Relationships of Amastrince. 

 Of the several subgenera of Amastra, we consider Amastrella 

 nearest to the primitive progenitor of the group, because of 

 the simplicity or lack of specialization in any part of the 

 shell. All of the other groups have some special modifica- 

 tion of shape, columellar lamella, embryonic whorls, or cuticle, 

 which render it unlikely that any of them is so near to the 

 original stock. The wide distribution of Amastrella, from 

 Kauai to Hawaii a range greater than that of any other 

 Achatinellid group except Leptachatina is also favorable to 

 the theory that it is an old, unchanged group. Cyclamastra 

 differs from Amastrella only by its open umbilicus, is also 

 widely spread, from Kauai to Maui, and is apparently an 

 early branch of Amastrella, the two being collateral phyla. 



The Kauaian groups Kauaia, Armiella and Carelia stand 

 much closer to Amastrella than to any other group, and have, 

 in our opinion, been derived from Amastrellan ancestors. If 

 so, all Kauian Achatinellidce have been evolved from a primi- 

 tive fauna composed of Leptachatina, Amastrella and Cycla- 

 mastra. 



In the island of Hawaii, Achatinellidce are almost entirely 

 confined to the old northern portion of the island, where the 

 family is represented by two groups of Amastrella, and a 

 fossil species of Heteramastra. The few arboreal Achatinellas 

 are related to Mauian forms. 



The intermediate islands fall into two groups (a) Oahu, 

 and (&) Molokai, Lanai and Maui. The fauna of these three 

 islands is so homogeneous that there cannot be much doubt 

 that they were formerly united into one large island. The 

 'Chance that so many groups could attain a foothold on the 

 three islands by adventitious means is so remote as to be 

 negligible. It is >on these two groups of intermediate islands 

 that the Achatinellidcu reach their greatest development in 

 numbers and diversity. Leptachatina, Cyclamastra, Amas- 



