INTRODUCTION. XIX 



IV. In the eastern end of this Oahu-Maui island Laminella 

 arose from Amastroid, and in the west, Pterodiscus was 

 evolved from Cyclamastra stock. 



V. (Fig. 3.) The Oahuan and the Molokai-Lanai-Mauian 

 areas were sundered by subsidence of the Kaiwi Channel. 



(a) In Oahu there were two centers, probably two islands, 

 a western or "Waianae, and an eastern or Koolau area. In 

 the Waianae center, Paramastra, Planamastra and Armsia 

 were differentiated, while Metamastra arose from Amastrella 

 in the Koolau area, where arboreal Achatinellidce chiefly flour- 

 ished. In late Pliocene or Pleistocene time a forested con- 

 nection was established between the two Oahuan evolution- 

 centers. This transitory connection allowed some inter- 

 mingling of the two faunas; but while the land connection 

 endured to the present time, the forests became extinct, again 

 isolating the two centers so far as arboreal or forest snails 

 are concerned. 



(&) That the eastern or Molokai-Lanai-Maui region formed 

 a single large island up to late Pliocene or even, to Pleistocene 

 time is evident from the very close relationship of the faunas 

 of those islands. In view of the intense local differentiation 

 everywhere observed in the Hawaiian group, we could hardly 

 expect closer relations between the species of these three 

 islands if they were still united. Various Achatinellinse of 

 Partulina type, the typical group of Amastra (found nowhere 

 else), the subgenus Heteramastra and the genus Laminella, 

 are the chief groups of this area. Probably all had been 

 differentiated before the separation from Oahu, though part 

 of them never spread, apparently, so far as that island. The 

 formation of channels between Molokai, Lanai and Maui must 

 be considered a very recent event. These islands stand on a 

 common platform within the 100-fathom line (see fig. 4, rep- 

 resenting the present islands, the 100-fathom contour in 

 broken line). 



