172 PROCEEDINGS OF THE ACADEMY OF [1883. 



the genus Tricula of Benson with the Melaniidae, an arrangement ' 

 which has been followed hy Chenu.^ 



Benson's Tricula is based upon a small fluviatile fonn which 

 the Adams say " somewhat resembles Paludomus ; * * * * 

 the only species known is an inhabitant of the River Kamaan in 

 India." The specific name montana implies a station similar to 

 those inhabited by the various species of Pyrgula herein quoted. 

 The figure of Tricula as given by the Adams and Chenu, together 

 with the totality of testimony furnished by said authors, leads me 

 to suspect that the Indian species should be removed from the 

 Melaniid£e to the Hydrobiinse and near to Pyrgula. 



It is not without some little hesitation that I place the Sierra 

 Nevada shell in the genus Pyrgula. Its principal characteristics, 

 however, indicate said group as well as the environmental features. 

 Stimpson's generic description of Tryonia applies only to shells 

 longitudinally sculptured (" ribbed or plicated "), a too restricted 

 limitation for a generic standard in this case, because if literally 

 applied it would exclude ninety-five per cent, of the individuals 

 which form the mass of which Stimpson's ^ species is but a rare 

 varietal aspect. Upon this point he wrote : " In company with 

 the Trj'onise, Mr. Blake found a small cancellated shell, which has 

 been described as Blelania exigua by Conrad, and as Amnicola 

 protea hy Gould. In view of the character of the surface, I think 

 it scarcely possible that this species can belong to the Hydrobiina?. 

 It will, perhaps, be found to be allied to Bittium. The occurrence 

 of this marine or brackish-water genus in the Desert would not 

 be surprising, since Gnathodon was found in the same basin at a 

 point somewhat nearer the Gulf." It is quite evident to my mind 

 that Stimpson could not have had a very large number of specimens 

 as they are usually found ; if so, they would have included not 

 only his T. clafhrata^ as well as Conrad's and Gould's types, but 

 intermediate and connecting varieties, sufficient to have caused 

 him to expand his generic diagnosis, and either to have made him 

 hesitate before investing the variety before him with specific 

 dignity, or else to have included Conrad's and Gould's forms as 

 species of Tryonia. He was not aware of the countless millions 

 of these tiny shells, that are scattered over a vast area, or of the 

 depth of the fresh-water sedimentarj' deposit throughout which 



1 Manuel de Couchyliologie, etc., p. 294, vol, i. 

 " Researches, etc., etc., id., p. 48, et »eq. 



