194 PALAEONTOLOGY OF CALIFORNIA. 



marked posteriorly, the upper tooth on that side extending for 

 nearly half the length of the hinge. 



Figure, slightly magnified. 



Localities: Division A., Cottonwood Creek, Shasta County ; also Tuscan Springs, 

 Tehama County. 



A. Hornii, n. s. 



PI. 30, Fig. 263. 



Shell minute, subequilateral; beaks subcentral, approximated; 

 anterior end broadly rounded; basal most prominent below, or, a 

 little in advance of the beaks, nearly straight posteriorly, and 

 inclined upwards towards the posterior, which is obliquely trun- 

 cated; posterior side abruptly truncated behind the umbonal 

 angle ; area short and very narrow. Surface marked by very 

 fine, radiating strise, somewhat undulated laterally, and crossed 

 by still finer lines of growth, with an occasional coarser line 

 formed by a slight interruption in the growth. 



Figure, magnified between three and four times. 



Locality: This beautiful little shell appears to be quite abundant in the vicinity 

 of Fort Tejon, from which place Dr. Horn has sent me numerous specimens. 



A. GRAVIDA, 11. 8. 

 PI. 30, Fig. 264. 



Shell thick, oblique, convex, very inequilateral; beaks small, 

 approximate, placed about one-third of the length from the ante- 

 rior end ; base broadly rounded, inclined upwards from the pos- 

 terior end, and convexly truncated in advance; posterior end 

 very obliquely truncated; umbonal ridge very prominent and 

 rounded ; posterior to this the surface is nearly flat and at a right 

 angle with the rest of the shell ; area short and narrow. Hinge 

 composed of about half a dozen small, transverse teeth under the 

 beaks, three or four large lateral ones in front, and five or six 

 behind. Surface apparently plain, or marked only by fine lines 

 of growth. 



