164 PALAEONTOLOGY OF CALIFORNIA. 



species, since I collected numerous specimens myself, at the original locality ; 

 and this is the only species with the peculiar surface ornamentation, mentioned 

 by Mr. Conrad, that has been found in California. 



M. LENS, n. S. 

 PL 23, Fig. 143. 



Shell compressed, lenticular; outline variable, subelliptical ; 

 beaks small, placed about two-fifths from the anterior end, which 

 is broadly convex; posterior end narrower; cardinal margin 

 curved, gradually sloping ; basal regularly convex. Surface 

 marked by a few irregular lines of growth. 



Figure, natural size. The species sometimes grows a third larger. 



Locality : Chico Creek, Butte County (Division A.). 



The outline of this shell is rather more variable than is usual in the genus. 

 The specimen figured is 1.15 inch long and .9 inch wide ; another is 1.2 inch long 

 and 1.1 wide. The convexity also is not constant, old specimens being propor- 

 tionally deeper than young ones, and often specimens of the same size will vary. 

 The average depth of a single adult valve is about .2 inch. 



M. Hornii, n. s. 



PI. 23, Fig. 144. 



Shell subtrigonal, cuueate ; beaks anterior, with the cardinal 

 margin sloping convexly to the posterior end, which is narrow ; 

 anterior end prominent above, and curving inwards rapidly below 

 towards the base. Lunule small, very narrow, acute below and 

 impressed. Surface marked by prominent, large, concentric 

 ribs, with interspaces of about the same width. 



Figure, natural size, from a small specimen. 



The species grows to about one-third larger than the figure. The artist has, to 

 some extent, misrepresented the characters of this shell. It should be more acute 

 posteriorly, the anterior end should be higher above and more oblique below, and 

 the concentric markings are at least a third too numerous. Judging from the 

 figure, there would seem to be little if any difference between this shell and M. 

 araia, fig. 250 ; but the triangular form, much larger size and distinct surface- 

 characters cannot fail to distinguish them. 



Locality: Near Fort T£jon. Dr. Horn. 



