CRETACEOUS FOSSILS. 193 



its gradual decline before its final disappearance, are the most apt to present varia 

 tions from the normal type of the genus. 



INOCERAMUS, Sby. 

 I. ELLIOTII, n. a. 



PI. 81, Fig. 90 a. 



SHELL large, equivalve, subquadrate; beaks terminal ; cardinal 

 margin Btraight,(?) nearly as long as the shell; anterior end trun 

 cated, nearly straight; posterior end straight and sloping out 

 wards above, rounded below, and, with the base, forming a nearly 

 perfect semicircle. Surface marked by about a dozen large, sharp, 

 concentric, imbricating ribs or lamellae, with broad concave inter 

 spaces. 



Figure, natural size ; from a cast in sandstone: the accompanying outline sec 

 tion of the surface is restored from impressions in the matrix. 



From the semi-metamorphosed Cretaceous sandstone on Alcatraz Island, San 

 Francisco harbor. Presented to the survey cabinet by Major George H. Elliot, 

 U. S. Engineers, by whom the first specimens were discovered. On subsequently 

 visiting the locality, I found numerous casts of this and of several other bivalves, 

 the latter in too imperfect a condition to be recognized. The species is of unusual 

 interest, being the first incontestable proof discovered, of the Cretaceous age of the 

 long-disputed "San Francisco Sandstone." 



I. WHITNEYI, n. s. 



PL 32, Fig. 91. 



SHELL large, elongate, subelliptical; beaks large, prominent, 

 anterior, subterminal; cardinal margin straight; anterior end 

 and base broadly, regularly, and continuously rounded ; posterior 

 end forming a curve of longer radius, but also continuous with 

 the base. Surface marked by about sixteen large concentric ribs, 

 the basal margin being abruptly truncated at a nearly right angle 

 with the remainder of the surface ; details of surface ornament 

 unknown. 



PAL. VOL. II. 26 



