PTERODISCUS. 119 



for Helix alata Pfr. Not Tropidopterus Blanchard 1845 

 (Coleoptera). 



Amastrinre with the shell lens-shaped or planorboid, much 

 wider than high, umbilicate, thin, fragile, dull brown, composed 

 of few (3y 2 to 5) whorls, which are carinate or angular per- 

 ipherally at all stages of growth; peristome thin, or fragile, 

 the colurnellar margin dilated, elsewhere simple. A coin- 

 mellar lamella is present at all stages of growth. The em- 

 bryonic shell, of about 1% whorls, is flat above, conically 

 tapering below, with microscopically crinkled cuticle and 

 sculpture of weak, irregular radial wrinkles chiefly on the 

 last half whorl. The adult shell is often loaded with for- 

 eign material adhering chiefly in the form of an irregular 

 extension of the periphery. Viviparous. 



Type : P. wesleyi Sykes. Distribution : Lanai and Oahu, 

 living on the ground. 



Pterodiscus was clearly derived from Amastra, agreeing 

 with that group in the dentition, the color and texture of 

 the shell and in habits. 



Pterodiscus differs from Planamastra and Armsia by the 

 presence of a columellar lamella at all stages of growth, from 

 embryos of one whorl to the adult; but in some adult in- 

 dividuals of P. wesleyi the lamella is very much reduced. 

 This reduction does not indicate close relationship between 

 P. ivesleyi and Planamastra, since the process has been in- 

 dependent in the two forms. The immediate ancestors of 

 Pterodiscus wesleyi doubtless had a well-developed columel- 

 lar lamella, whereas in Planamastra the lamella was lost so 

 long ago that no trace of it remains even in the embryonic 

 stage. 



All of the species of Pterodiscus usually plaster foreign 

 matter, perhaps in part excrement, upon the shell, where it 

 adheres chiefly at the periphery in form of an irregular 

 flange. Some individuals are clean. Certain Amastras have 

 the same peculiarity, but it has not been observed in Plana- 

 mastra or Armsia. The shells of Pterodiscus are so thin 

 that it is difficult, to handle them without breaking. 



This genus was first proposed under the name Tropidoptera 



