120 PTERODISCUS. 



by Mr. C. F. Ancey, who placed it between Sitala and 

 Hycdinia, assigning to it one species, Helix alata Pfr. This 

 generic terra being preoccupied, Pilsbry substituted the name 

 Pterodiscus, transferring the group to the Endodontida as a 

 subgenus of Endodonta. The type of Pterodiscus was stated 

 to be H. alata Pfr., but the specimen so identified and figured 

 by Pilsbry proved to be not the alata of Pfeiffer, but a new 

 species which Mr. Sykes subsequently named P. iveslcyi. 

 Sykes raised Pterodiscus to generic rank in 1900 ; and in 

 1905 Pilsbry and Vanatta showed it to belong to the Aclia- 

 tinellidce near Amastra. At the same time they created a 

 group Helicamastra, which is now found to be superfluous. 

 Professor Hyatt, in his MS. on Amastrince recognized P. 

 cooJcei and P. heliciformis as representing a new genus allied 

 to Amastra, but no other species was known to him. The fol- 

 lowing accounts of Pterodiscus, Planamastra and Armsia 

 have therefore been prepared wholly by Pilsbry. 



The embryonic shell has been fully described under P. tlia- 

 anumi and P. a. litus. 



Key to Species of Pterodiscus. 



a. Species of Lanai. 



&. Umbilicus one-sixth the diam. of shell. 



P. alatus, no. 1. 



ft 1 . Umbilicus one-eighth the diam. P. a. litus, no. la. 

 a 1 . Species of Oahu. 



&. Spire flat, or convex with flattened apex. 



c. Upper surface flat; umbilicus contained 5^ 

 times in the diameter; periphery obtuse; 3.4x 

 10 mm. ; Waianae. P. discus, no. 2. 



c 1 . Upper surface slightly convex; umbilicus con- 

 tained 4 times in diam.; columellar lamella 

 subobsolet2 ; 3.5x8.2 mm. ; Kalaikoa, Wahiawa. 



P. wesleyi, no. 3. 

 c 2 . Similar but col. lam. distinct; Ewa. 



P. w. ewaensis, no. 3a. 

 c 3 . Upper surface convex ; umbilicus nearly 5 times 



