1'LACOSTYLUS, SOLOMON IS. 77 



SPECIES OF THE SOLOMON ARCHIPELAGO. 



The species of the Solomons fall into three moderately well-char- 

 acterized groups : 



1. P/acocharis, including ovate, smooth or spirally corrugated 

 species, with plain brown or variegated cuticle, 2^ pitted nepionic 

 whorls, and a moderate or strong columellar fold. Type P. mac- 

 gillivrayi. 



2. Aspastus, with the shell white, elongated and thin, without 

 noticeable spiral sculpture ; 2-| pitted nepionic whorls, the columellar 

 fold very weak, not calloused. Arboreal. Type P. miltocheilns. 



3. Eumecostylus, having the shell much lengthened, the spire slen- 

 der, with 877 pitted nepionic whorls, the columella ascending with a 

 long strongly spiral trend. Type P. cleryi. 



From the phylogenetic point of view, all of these are doubtless 

 more nearly related inter se than any of them to forms of other 

 islands ; Aspastus and Eumecostylas being derivatives from the 

 Placocharis group, which consists of generalized forms not much 

 modified from the ancient common stock of all the archipelagos. 

 The differentiation of Aspastus and Eiimecostylus has probably been 

 since the period when the islands were united into a single land mass, 

 supposing that to have been once their condition, as the species of 

 these sections are confined to the two southern islands of the group, 

 while the parent stock, Placocharis, is spread over the whole archi- 

 pelago. 



All of the species are confined to single islands or their contiguous 

 islets ; and as the larger islands are but little known, and some of the 

 smaller wholly unknown malacologically, there will doubtless be 

 many more species of Placostylus found in the group. San Chris- 

 toval Island posesses the most divergent forms, perhaps indicating 

 more prolonged isolation than the other islands. 



The distinction between arboreal and terrestrial species is less 

 marked in the Solomon Islands than in New Caledonia. 



Key to Species. 



A. Surface densely corrugated spirally. 

 a. With maculate color pattern. 



b. Fusiform, 3^ early whorls punctate ; yellow with 

 brown or olive maculation, lip more or less red ; fold 

 moderate. sanchristovalensis, p. 97. 



