192 PALAEONTOLOGY OF CALIFORNIA. 



terior margin is abruptly thickened, both externally and inter- 

 nally. This latter character is very variable even in specimens 

 of the same size and from the same locality; the thickening 

 being, in some specimens, twice as great as in others. Surface 

 marked only by lines of growth. 



Figure, natural size. 



Localities: Abundant near Martinez (Division A.). Also found at Tuscan 

 Springs, Tehama County, and in the Siskiyou Mountains. 



M. RADIATA, 11. S. 

 PL 25, Fig. 179 a. 



This little shell is almost a perfect miniature of the preceding 

 species ; but the points of difference, although small, seem to be 

 constant, and I have ventured to elevate it to the rank of a dis- 

 tinct species, despite my first impressions. 



The shell is proportionally more gibbous, the beaks are more 

 prominent, the anterior end is more produced, the whole outline 

 is less quadrangular, and the anterior fourth of the surface is 

 marked by a few radiating lines. The most marked difference 

 between the two forms is, that this one, although found at a 

 number of localities, over a range of about five hundred miles, 

 and in some places quite abundantly, never attains a size greater 

 than the figure, which is from an unusually large specimen 



Localities: From Division A., at the Rancho de San Luis Gonzaga, in Pacheco's 

 Pass; Orestimba Canon, Stanislaus County; Tuscan Springs, Tehama County; 

 Siskiyou Mountains, Siskiyou County, and at Jacksonville, Oregon. 



M. navis, n. s. 



PI. 25, Fig. 180. 



Shell long, boat-shaped, thin ; beaks small, central, not pro- 

 minent ; cardinal margin concave in front, convex and slightly 

 sloping behind; anterior end angular above, sloping inwards, 

 rounded below ; posterior end regularly rounded ; base slightly 

 convex, nearly straight in some specimens. Surface marked only 



