148 PALEONTOLOGY OF CALIFORNIA. 



PANOP^EA, Menard. 



P. CONCENTltICA, U. S. 

 PL 22, Fig. 119. 



Shell subquadrate, about a third longer than wide; beaks 

 small, about a third of the length from the anterior end. Poste- 

 rior cardinal line very slightly sloping ; basal margin usually regu- 

 larly curved, sometimes nearly straight. Ends gaping; posterior 

 end widest. Surface marked by rather large, irregular, concen- 

 tric ribs. 



Figure, natural size. Sometimes the species attains twice this size ; and one 

 specimen in the collection of the California Academy of Natural Sciences, from 

 Tuscan Springs, which probably belongs to this species, has a length of four and 

 a half inches. 



Localiiy: Cottonwood Creek and Martinez (Division A.). 



CORBULA, Brug. 



? C. PRIMORSA, U. S. 

 PL 22, Fig. 120, and 120 a. 



Shell small, thin, abruptly truncated posteriorly ; beaks nearly 

 central, small ; anterior end produced, broadly rounded, excavated 

 below the beaks; a well-marked angulation extending from the 

 umbones to the posterior basal angle. Surface marked by nume- 

 rous fine, radiating lines, and by coarser, irregular lines of growth. 



Length, .37 inch; width, .25 inch. 



Figure 120 a is from the posterior end. 



Localiiy : South side of Corral Hollow, Alameda County. Probably Cretaceous. 

 I have not been able to uncover the hinge of the only specimen I have seen, 

 and therefore refer this species to the genus Corbula with some doubt. 



