166 PALAEONTOLOGY OF CALIFORNIA. 



M. ARATA, 11. S. 

 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 CaSon, 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. 8. 

 PI. 30, Fig. 251. 



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

 tral ; anterior margin broadly convex, produced, most prominent 

 above the middle, subtruncated 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. Californica, Con. Pacific K. K. 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 



