PTERODISCUS. 127 



within. The second whorl is weakly or distinctly margined 

 above the suture. The striation is very fine and close, with- 

 out coarse wrinkles. Columellar lamella is well developed 

 and horizontal. Some specimens have a flat wing with tri- 

 angular processes built out upon the peripheral keel. Alt. 

 3.5, diam. 9 mm., with S 1 /^ whorls. 



6. P. COOKEI Hyatt & Pilsbry, n. sp. PL 23, figs. 6, 7, 8. 



The shells are exceedingly thin and fragile, narrowly um- 

 bilicate, the umbilicus contained GI/> to 9 times in the diameter 

 of the shell ; biconvex, sometimes incrusted at the periphery 

 and umbilicus. It is dull brown, with sculpture of fine re- 

 tractive stria? and coarse, low wrinkles in the same direction. 

 The 1/2 whorls are very slightly convex, the last angular 

 at the periphery, convex beneath, not angular around the 

 umbilicus. The suture is well impressed, but has no mar- 

 gination above on the second whorl. The columella is di- 

 lated, thin and paper-like, brownish. The columellar la- 

 mella is very weak on the dilated part of the columella. but 

 inwardly it becomes somewhat stronger, though thin and 

 bent downwards, Alt. 6, diam. 11 mm. ; sometimes decidedly 

 more depressed. 



Oahu: Nuuanu valley (Cooke). Type in coll. C. M. Cooke. 



This excessively fragile species is probably most closely 

 related to P. rex, from an adjacent locality, but it seems to 

 differ by the weaker columellar lamella and greater fragility. 

 It is also less depressed, the lowest shell measuring about 

 4.5 x 10.5 mm., so far as can be ascertained in its broken 

 condition. It is remarkable for the absence of margination 

 above the suture on the second whorl, though there is a barely 

 perceptible flattening in that region. Mr. Sykes does not 

 mention whether the second whorl has a suprasutural mar- 

 gination in P. rex. 



7. P. HELICIFORMIS ( Ancey) . PI. 36, fig. 10. 



"Shell heliciform, depressed, broadly umbilicate (the um- 

 bilicus open, deep, surrounded by an angle, about 2 mm. 

 across), hardly shining, brown, wrinkle-striatulate, a little 



