12 SPIRAXIS. 



before the separation of the Antillean and mainland 

 areas. The subgenus Volutaxis contains the more primitive 

 forms. It occurs in both areas, though sparingly in the 

 Antilles. In Jamaica the most evolved forms are found, the 

 restricted subgenus Spiraxis representing a gerontic phylum. 

 In another Jamaican series, Sigmataxis, the shell has varix- 

 lines and resembles Varicella except in its columella. The 

 exact rank and relationships of the several Spiraxoid phyla, 

 here treated as subgenera, must depend upon the morphology 

 of the soft parts, of which we know practically nothing. 

 There is, however, every reason to believe that the sigmoid 

 columella in Spiraxis, as in Streptostyla, Rectoleacina etc., 

 was derived from a truncate, Aehatinoid columella. A simi- 

 lar transformation has been noted in several groups of 

 Acliatinidce. 



Spiraxis was proposed for the species inusitata Ad., aber- 

 rans (Pfr. ?) Ad., and costulosa Ad. The diagnosis applies 

 particularly to the first and last of these species, while the 

 second fits it only imperfectly. Adams' first species is there- 

 fore taken as the type of Spiraxis. The limits of the genus 

 were only imperfectly perceived by Adams, but he saw clearly 

 the necessity for a genus to contain Jamaican species which 

 were not referable to Bulimus orAchatina as those groups- 

 were then limited. Shuttleworth, in 1852, enlarged Spiraxis 

 to cover Streptostyla, Columna and Clavator, which have since 

 been segregated as distinct genera, but which have in com- 

 mon a spirally twisted and not truncate columella. In as- 

 sociating Spiraxis with the Oleacdnoid Streptostyla, Shuttle- 

 worth did not go far astray, although we now understand the 

 resemblance in columellar structure to be due to convergent 

 modification in the two groups. It was a step backward when 

 Spiraxis was placed among the Stenogyroid snails. 



Numerous species now placed in other groups were origin- 

 ally described as Spiraxes. 



The following belong to STREPTOSTYLA: Spiraxis auricu- 

 lacea, biconica, boucardi, catenata Pfr., cobanensis Tristr., 

 coniformis Sh., dubia Pfr., flavescens, irrigua, lurida, limnei- 

 formis, mitriformis Sh., mohriana Pfr., nicoleti Sh., oblonga, 



