GASTROCOPTA, NORTH AMERICA, WEST INDIES. 13 



Keys to the species will be found under the subgeneric 

 heads. 



Subgenus ALBINULA Sterki. 



Albinula STERKI, Nautilus, vi, 1892, pp. 4, 101, type P. 

 contracta. Leucochilus BOETTGER, Conchol. Mittheil., i, 1881, 

 p. 64, type Pupa armifera. Not Leucochila von Martens, 

 1860. 



Whitish-translucent Gastrocoptas having the inner end of 

 the parietal lamella curved towards the periphery ; angular 

 lamella well developed, coiicrescent in varying degree with 

 the parietal; the palatal plicse stand upon a white palatal 

 callous, and a suprapalatal plica, is usually developed. Ex- 

 cept in G. armifera, the columellar lamella is horizontal in 

 front and curves towards the base within. The lip is thin 

 and expanded. 



Type G. contracta (Say). Distribution, North America 

 except the Pacific slope ; Oligocene to Pliocene of Europe. 



If names differing from prior generic terms only in gender 

 are acceptable, Leucochilus will replace Albinula for this sub- 

 genus. 



In America, the species of Albinula are few and strongly 

 differentiated from one another by the structure of the pari- 

 etal and columellar lamellse, which can be seen fully only by 

 breaking away the back or base of the shell. Albinula, is 

 peculiar By the usual development of a suprapalatal plica; 

 a fold which is never found in Immersidens or the typical 

 group of Gastrocopta. It is related to Sinalbinula and its 

 derivative Vertigopsis, but had already been differentiated 

 from these groups in the Oligocene, when the group first 

 appears. The very small number of recent species of Albi- 

 nula (3), and the fact that in Europe the group became ex- 

 tinct after a long series of mutations without much structural 

 change, probably indicates that the subgenus is relatively 

 inflexible and inadaptive, and now iiears final extinction; 

 yet two of the recent species are widely spread and abundant 

 snails in eastern North America. 



Immersidens and Gastrocopta (restricted) extend from the 



