PTERODISCUS. 123 



The umbilicus is decidedly narrower than in Dr. New-comb 's 

 specimens of H. alata. The embryonic shell when fully de- 

 veloped is 2.5 mm. in diameter, with about 1% whorls. The 

 first whorl is slightly convex, smooth except for a dense 

 microscopic crinkling or shriveling of the cuticle; the next 

 whorl is radially rather coarsely striate. There is no ap- 

 pearance of spiral striation due to revolving bands of 

 microscopic wrinkles, such as is seen in P. thaanumi. 



2. P. DISCUS (Pilsbry & Vanatta). PL 22, figs. 7, 8, 9. 



Shell thin and fragile, openly umbilicate, flat above, con- 

 vex below, the periphery carinate, the keel obtuse. Uniform 

 dull brown. Whorls nearly 4, the first hardly convex, marked 

 with faint growth-lines; second whorl flat, weakly marked 

 with oblique growth-lines, its outer edge distinctly margined, 

 cord-like. The next whorl is slightly convex, without a periph- 

 eral cord or margin, and has rather rude wrinkled striae. 

 The last whorl becomes more swollen above, in part rising 

 above the level of the preceding whorls, and is rather rudely 

 wrinkled. The angular periphery is on a level with the flat 

 spire. The umbilicus is deep and subcylindric. The aper- 

 ture is oblique, wider than high; outer and basal margins 

 thin and simple, columellar margin dilated, bearing an acute, 

 subhorizontal white lamella. Alt. 3.4, diam. 10 mm. 



Waianae, Oahu. Types No. 58,158, A. N. S. P., received 

 from D. D. Baldwin. 



Amastra (Helicamastra} discus PILS. & VAN., Proc. A. N. 

 S. Phila. 1905, p. 571, pi. 38, f. 1-3. 



This curious snail differs from P. alatus by its flat spire, 

 irregular growth and larger umbilicus. It is much more de- 

 pressed than P. heliciformis Anc., with fewer whorls. 



Pterodiscus rex Sykes is a larger species with peripheral 

 appendages as in Pterodiscus wesleyi. It seems to have a 

 much smaller umbilicus than P. discus, but neither the de- 

 scription nor figure are clear on this point. 



3. P. WESLEYI (Sykes). PL 23, figs. 1-5, 9. 



Shell umbilicate, thin, flat above, convex beneath, carinated 



