﻿GONIDEA 465 



Anodon biangulata Sowkruv, Conch. Icon., XVII, 1869, pi. 



xxin, figs. 8, Sa, Sb. 

 Anodonta biangulata Ci.essin, Conch. Cab. Ano., 1876, p. 234, 

 pi. Lxxvii, fig. 3. 



An exceedingly variable and apparently aberrant form. In 

 most specimens the posterior ridge is the most striking char- 

 acter, being high, sharp and often pinched up. Occasionally 

 it is lower and somewhat double and rarely it is scarcely de- 

 veloped at all. Sometimes the species lives in rapidly running 

 water, the anterior part buried in the hard bottom as far as 

 the posterior ridge, which runs on a level with the surface of 

 the bottom, the posterior end pointing down stream. It is 

 probable that specimens living in such environment have a 

 strong, high posterior ridge and that those found in compara- 

 tively still water have it much less developed. I have a photo- 

 graph sent by Mr. Henry Hemphill of a shell in which the 

 posterior ridge is almost wanting and such specimens are 

 usually thinner than those with the high ridge, an evidence of 

 living in stiller water. The form of the shell, the habits of 

 the animal and its large foot would hint at a relation with 

 Solenaia. 

 Var. snbangulata (Hemphill). 



•'In general outline this form recalls at once its nearest rela- 

 tive, A. angidata Lea. Compared with that species, it is more 

 oval in outline ; the ridge or angle radiating from the umbones 

 to the anterior (posterior) ventral margin of the shell is nearly 

 or quite obsolete in the specimens before me; the anterior 

 margin of the shell is more produced on its dorsal edge, while 

 its ventral edge is more rounded at the point where the angle 

 terminates, and the intervening space between the angle and 

 the front margin of the shell is less concave in form. The 

 shell is also less pointed and more rounded in its posterior out- 

 line, and is much less inflated in general form. In color and 

 lines of growth the two shells agree fairly well 



Compared with Anodonta oregonensis Lea, its next nearest 

 relative, our variety is just the reverse in outline. It is broader 



