20 PALAEONTOLOGY OF CALIFORNIA. 



Localities: Common in the Miocene, south of San Pablo Bay; south of Marti- 

 nez; near Walnut Creek, and in the hills east of Oakland, Contra Costa County; 

 San Emidio Eanch ; west of Fort Tejon (and Santa Barbara, Con.). 



From its extreme thinness, this shell is almost always found very much dis- 

 torted. The specimen figured by Mr. Conrad had been pressed out of its natural 

 shape, and probably, having been misled by this inconstant form, he has con- 

 founded two species, widely different in character, under the same name. 



SCHIZODESMA, Gray. 



S. ABSCISSA, 11. S. 

 PI. 4, Fig. 34, 34 a. 



Shell large, thick, irregularly subquadrate ; beaks large, pro- 

 minent, pointed forwards, nearly central; anterior end obliquely 

 truncated; base broadly rounded; posterior end very strongly 

 and abruptly truncated, the truncation ending at an angular 

 ridge which passes from the beaks to the posterior basal angle. 

 Surface marked by rough, irregular lines of growth. Hinge 

 composed of large, robust teeth; lateral teeth long and thick. 



Figures, natural size. Fig. 34 a, represents the posterior end. 



Localities: South of Martinez ; near San Pablo Bay and Walnut Creek, Contra 

 Costa County ; Miocene. 



From the character of the hinge, and the fact that this and the preceding species 

 are often found associated, it is probable that this is the species which Mr. Conrad 

 confounded with his Mulinia densata ; although the shape of the shell precludes 

 the possibility that this is the form to which he intended to apply the name. 



PSEUDOCARDIUM, Gabb. K Gen. 



Shell thick, heavy, equivalve, resembling Jjcevicardium exter- 

 nally; ligament internal. Lunale cordate, deeply impressed. 

 Hinge composed of a large cartilage pit and in the left valve, a 

 V-shaped tooth articulating in a corresponding depression on the 

 opposite valve; two lateral teeth in each valve, very large and 

 prominent; these teeth are comparatively narrow in the young 



