166 PALAEONTOLOGY OF CALIFORNIA. 



M. ARATA, 11. 8. 

 PI. 30, Fig. 250. 



Shell thick, rounded-subtriangular, nearly as high as long; 

 beaks large, prominent, a third of the length from the anterior 

 end, which is narrowly excavated above, rounded below, and 

 broader than the posterior extremity ; base regularly convex ; car- 

 dinal margin curving regularly to the anal end. Lunule small. 

 Surface ornamented by regular, concentric, acute, impressed 

 lines. 



Figure, natural size. 



Localities: Cottonwood Creek, Shasta County ; Siskiyou Mountains ; and Ores- 

 timba Caiion, Stanislaus County. 



Allied to M. nitida in form, but is less produced in advance, more prominent 

 behind, and is easily distinguished by the well-marked surface ornamentation. 



M. OVALIS, 11. S. 

 PI. 30, Fig. 251. 



Shell oval, convex, rather thick; beaks small, acute, subcen- 

 tral ; anterior margin broadly convex, produced, most prominent 

 above the middle, sub truncated below ; cardinal margin regularly 

 curved, sloping downwards posteriorly, uniting with the posterior 

 end, which, from lines of growth, seems to have been faintly trun- 

 cated. Surface marked only by lines of growth. 



Figure, natural size. 



Locality : A single specimen, found near Fort Tejon by Dr. Horn (Division B.). 



This species appears to be closely allied to M. lens, from Chico Creek, in Divi- 

 sion A. It is, however, a thicker, more robust shell, is more convex ; the beaks 

 are larger and are of a different shape, and the shell is altogether more regularly 

 elliptical. 



M. Californica, Con. 



(M. California, Con. Pacific Pv. Pv. Rep., vol. 5, p. 320, pi. 2, fig. 4.) 



Although I searched thoroughly the locality from which this species was re- 

 ported, and spent several days collecting fossils there, and although Dr. Horn has 



