186 PALAEONTOLOGY OF CALIFORNIA. 



characters are certainly of sufficient value to separate the group as a distinct 

 genus, which, in addition to the living species, comprises numerous fossils in the 

 Eocene, and some in the Cretaceous. 



C. nitida, Gabb. 



PI. 30, Fig. 79. 



(Meretrix nitida, Gabb; Pal. Cal., Vol. 1, p. 1G5, pi. 23, fig. 145, 146.) 



A fine shell, very variable in form in different stages of growth, and character- 

 istic of the Chico Group. The present figure is from a young shell, from Martinez. 

 The largest specimen I have seen is nearly two inches in length. 



THETIS, Sby. 



? T. ELONGATA, 11. S. 

 PI. 30, Fig. 80, 80 a. 



Shell small, very thin, subquadrate, gibbous, elongate; beaks 

 large, incurved, a fourth of the length from the anterior end; 

 cardinal margin sloping and nearly straight; anterior end deeply 

 excavated under the beaks, rounded below, and merging by a 

 regular curve into the base, which is broadly convex, and most 

 prominent in the middle; posterior end obliquely truncated ; a 

 strongly marked subangular ridge runs from the beaks to the 

 posterior basal angle, behind which the surface is slightly con- 

 cave. Surface marked by very minute lines of growth. 



Length, .52 inch ; width, .44 inch ; diameter through both valves, .38 inch. 



Not rare at a single locality in the vicinity of Cottonwood Creek, from either 

 the Chico or Shasta Group. Mr. Mathewson collected a considerable number of 

 specimens in 1865, from a gray rock, different from any bed with which I am 

 acquainted, and which seems to have yielded no other species. The shell is of 

 extraordinary thinness and delicacy, and is so transparent that, in some cases, its 

 presence can only be detected by the opacity of the thickened cardinal margin. 

 On account of its extreme delicacy, I have not even attempted the futile task of 

 exposing the hinge, and have referred it doubtfully to the above genus, from its 

 general appearance, rather than from any character that it possesses. 



