CRETACEOUS FOSSILS. 141 



HELCION, Montf. 



? II. CIRCULARIS, II. S. 

 PL 29, Fig. 234, and 234 a. 



Shell minute, subcircular, elevated, height about half the 

 length; apex suhcentral, in advance of the middle, slightly in- 

 clined forwards; sides sloping, concavely in front, convexly 

 behind, nearly straight on the sides. Surface marked by con- 

 centric undulations and lines of growth, and by numerous 

 dichotomous ribs, irregular in size. 



Length, .15 inch. 



Figures, magnified to the same scale. 



Locality: Near Martinez (Division A.); collected by Mr. Mathewson. 



I have seen but a single very perfect specimen of this species, and suspect that 

 it may be young. It closely resembles Anisomyon; but I have been unable to 

 detect any perforation of the apex, under a lens of high power, and the muscular 

 scar being hidden, it is, as yet, impossible to settle definitely its generic relations. 



H. DICHOTOMA, 11. S. 

 PL 21, Fig. 104. 



Shell subelliptical, obliquely conical; apex blunt, anterior, 

 nearly terminal. Surface marked by irregular concentric lines, 

 and small dichotomous ribs, obsolete towards the apex. 



Figures, natural size, and magnified. 



The higher and more anterior apex, and the small, very numerous, dichotomous 

 ribs, will distinguish this species from the preceding one. In P. Traskii, the spe- 

 cimens, when eroded, show no trace of the external ribs ; and small specimens 

 of that species are liable to be confounded with the present one, except for the 

 somewhat more central position of the apex. 



Locality: Texas Flat, Placer County. (Div. A.) 



