158 PALEONTOLOGY OF CALIFORNIA. 



Figures, natural size. 



Localities: Fig. 135 is of a specimen found west of Martinez ; 135 a, from Ponce's 

 Ranch, Butte County (Division A.). 



T. Mathewsonii, n. s. 



PI. 23, Fig. 13G. 



Shell equilateral, compressed; beaks broad, prominent, cen- 

 tral; extremities equal, or nearly so, broadly rounded; basal 

 margin regularly convex. Surface marked only by irregular 

 lines of growth. 



Figure, natural size. 



Locality: In Division A., in the neighborhood of MartiHez. The finest speci- 

 mens are from an outcrop about a mile north of the village of Pacheco, southeast 

 of MartiSez. 



I am indebted to Mr. Mathewson for the discovery of this species, and for the 

 specimens now in the collection of the Survey. 



T. DECURTATA, U. S. 

 PI. 23, Fig. 137. 



Shell moderate in size, compressed, very inequilateral; beaks 

 anterior ; cardinal margin sloping posteriorly gently, very slightly 

 convex ; anterior end abruptly truncated ; posterior end narrow, 

 rounded; base broadly and gently convex. Surface unknown. 

 Muscular impressions slight; pallia! sinus oblique, shallow and 

 round at the base. Primary teeth narrow, the anterior the largest; 

 lateral teeth small, almost obsolete. 



Figure, natural size. 



Locality: I found but a single specimen of this peculiarly shaped shell ; this was 

 obtained at Pence's Ranch, twelve miles north of Oroville, Butte County (Divi- 

 sion A.). 



Although I have but a single imperfect cast of this species, I do not hesitate in 

 describing it, in the confidence that its remarkable shape will not fail in distin- 

 guishing it from all known species. 



It approaches most nearly to T. ooides (supra) ; but, in addition to its more 



