PTERODISCUS. 17 



dirt-loaded shell assimilating well with their surroundings. 

 They are widely spread, and probably to be found with care- 

 ful search wherever conditions are suitable. As in the typi- 

 cal group of Endodonta, the colonies are isolated and prob- 

 ably small, though we suppose neither so small or so widely 

 separated as the colonies of Endodonta appear to be. The 

 incrusting dirt is, of course, foreign to the shell, to which it 

 adheres quite firmly, being cemented thereto by the slime of 

 the animal. Ptcrodiscus is colloquially known in the Islands 

 as " Flatamastra. " 



Having seen a good many specimens we are disposed to agree 

 with Mr. Spalding that too many Oahuan forms have been 

 described. They are rather variable in degree of depres- 

 sion, carination, width of umbilicus, and of course in the 

 amount of dirt carried. I had not time to go over Mr. 

 Spalding 's long series except in the most superficial way, but 

 a re-examination of the Academy 's series, augmented by seven 

 lots from as many localities, which I brought home, indicate 

 two species in each Oahuan range. 



1. Umbilicus narrower, often closed with agglutinated ma- 

 terial ; columellar lamella \vell developed : P. REX Sykes, in- 

 cluding P. thaanumi and P. cookei. Eastern in the Main 

 Range. 



2. Umbilicus wider, broadly conic, with angular margin; 

 columellar lamella smaller, receding: P. WESLEYI Sykes. 

 Western in the Main Range. 



3. Umbilicus narrow, columellar lamella well developed, 

 spire flat: P. DISCUS P. & V. Waianae Range. 



4. More elevated and solid, etc. : P. HELICIFORMIS Anc. 

 Waianae Range. 



P. heliciformis, hitherto reported merely from the 

 ''Waianae Range," has been found in some quantity in 

 Palikea (Thaanum). The specimens are somewhat smaller 

 than the types, and like them vary in elevation and size of 

 the umbilicus. Some of them, chiefly the young ones, are 

 conspurcate, especially at the periphery. 



