INTRODUCTION. xvii 



trella, Laminella and Pterodiscus are common to both the 

 Oahu and the Molokai-Lanai-Maui centers. 



(a) Oahu has two special groups, Metamastra and Para- 

 mastra, both believed to have evolved from Amastrella, and 

 the genera Armsia and Planamastra, derived from Cycla- 

 mastran stock. The scarcity of Laminella species may be due 

 to the competition of Achatinella, which is the dominant ar- 

 boreal group. 



(b) Besides the groups possessed in common with Oahu, 

 the Molokai-Lanai-Maui area has all of the typical section of 

 Amastra, all the Heteramastras except one Hawaiian species, 

 and nearly all of the genus Laminella. Probably all of these 

 groups were evolved from Amastrellan ancestors in this area. 

 Several groups of Acliatinellince are special to this tract. 



VI. 



From the affinities and the geographic relations of the 

 several groups, as sketched above and discussed in detail in 

 this work, we infer the following sequence of events, prob- 

 ably beginning in Mesozoic, possibly in Eocene, time : 



I. The Hawaiian area, from northern Hawaii to and prob- 

 ably far beyond Kauai, formed one large island which was 

 inhabited by the primitive Amastrinae, ancestral forms of 

 Leptachatina, Amastrella and Cyclamastra. This pan-Hawa- 

 iian land, whatever its structure, preceded the era of vul- 

 canism which gave their present topography to the islands, 

 and probably dates from the Palaeozoic (Fig. 1). 



II. Volcanic activity built up the older masses, subsidence 

 following, Kauai being the first island dismembered from the 

 Pan-Hawaiian area. The groups Carelia, Armiella and Ka- 

 uaia were subsequently evolved thereon from the Amastrellan 

 stock. Meantime the differentiation of Amastrine groups 

 from the primitive Amastrellae and Cyclamastrae began on 

 the larger land-area remaining (Fig. 2). 



III. Northern Hawaii was next isolated by formation of 

 the Alenuihana Channel, leaving a large intermediate island, 

 which included the present islands of Oahu, Molokai, Lanai 

 and Maui. 



