NEW FOSSIL MOLLUSCA OF CALIFORNIA. 333 



In describing this species the large specimen men- 

 tioned by Stanton was not seen by me nor recognized as 

 the adult of the small one figured, but a small single valve 

 like the figure was found, which had one tooth like that 

 of a Corbula, from which the genus was published, with 

 very much doubt. The description therefore needs 

 amendment to include the large specimen which is fully 

 twice the size of type. Its length is 0.75 inch at base, 

 breadth 0.55, anterior height 0.60. Beak forming an ob- 

 tuse point; transverse ribs over 35. It does not closely 

 resemble the figures of species of Opis given in textbooks, 

 and to show its resemblance to Corbula two better figures 

 of the type specimen are here given, with one of the adult 

 valve. In shape this closely resembles Stanton's O. cal- 

 if ornica in his pi. vii, fig. 3. 



Triplicosta n. subg. 



With the generic characters of Pholadomya (as far as 

 known) and similar in sculpture; the shell is not pearly 

 and thin, but dull and chalky; form like that of 

 Agassiz's section Multicosta? ; the ribs even more numer- 

 ous, covering the whole surface, and of two forms; the 

 posterior, simple, broad and rounded, cover about half 

 the surface, the anterior, formed by division of the broad 

 ones each into 3 or 4, occupy the rest, all diverging from 

 the beaks, with a slight curve backwards, to end at the 

 base. It thus approaches in richness of sculpture the only 

 living species, that of the West Indies, which has three 

 forms of ribs. While some of Agassiz's sections of the 

 genus are stated to be thicker-shelled than others, and 

 pearliness seems not to be universal, it is possible that 

 chalkiness may be caused by fossilization, as it is not 

 often mentioned in descriptions, especially of the West 

 Coast fossils. Though in most points best agreeing with 



