48 PALEONTOLOGY OF CALIFORNIA. 



Height, 1.17 inch: breadth, 0.97 inch; convexity (of right 

 valve only), 0.13 inch. 



I am equally in doubt in regard to the generic relations of this species, having 

 seen but a single specimen, which gives no idea of the nature of the hinge, or of 

 the form of the opposite valve. It differs from the last in being less oblique, 

 proportionally broader, and in having the beak less prominent, while its hinge 

 ranges more nearly at right angles to the longer axis of the shell. It is replaced 

 by a greenish choritic substance, showing no traces of the shell structure. 



FAMILY 



GENUS TRIGONIA, Bniguifcre, 1789. 

 TRIGONIA PANDICOSTA, Meek. 



SHELL small, rhombic subquadrate ? moderately convex; beaks 

 prominent, oblique, and nearly terminal ; anterior side very short, 

 rounding obliquely backwards into the rather short, rounded 

 base ; posterior side comparatively broad, compressed, and near 

 ly vertically truncated. Corselet wide, compressed, occupying 

 nearly half the surface of each valve, only marked with dis 

 tinct, regular thread-like lines of growth, separated from the 

 flanks by a linear ridge extending from the back part of each 

 beak, with a gentle curve, obliquely backwards and downwards 

 to the postero-basal margin. Flanks and anterior side orna 

 mented by six to eight costse, which extend from the margin of 

 the corselet, first obliquely backwards and downwards, after which 

 they are deflected very abruptly forwards and upwards. 



Length, about 0.55 inch; height, about 0.37 inch; convexity, 

 0.35 inch. 



Of this little Tngonia there is but a single specimen in the collection, and it is 

 so firmly embedded in the exceedingly hard matrix, that it is not possible to 



