62 PALAEONTOLOGY OF CALIFORNIA. 



Cottonwood Creek, Shasta County; Martinez and Clayton, and south of Mount 

 Diablo, Contra Costa County. Some fragments from Vancouver Island, in the 

 collection of the California Academy of Natural Sciences, indicate individuals 

 of a foot in diameter. 



A. Brewerii, n. s. 



PL 10, Fig. 7. 



Shell discoidal. Whorls four or live, sides slightly arched, 

 curving inwards towards the umbilicus, on the margin of which 

 it is angulated and abruptly truncated. 



Dorsum rounded. Surface marked by numerous rounded or 

 subangular flexuous ribs, which arise in the umbilicus, passing 

 obliquely forwards, cross the angle of the umbilical margin, 

 keeping nearly the same direction, then curving so as to cross 

 the middle of the whorl transversely; after which they again 

 curve forwards, usually becoming obsolete on the dorsum, 

 although, in one specimen I have seen, they retain their size 

 completely across the shell. 



Septum unknown. 



The figure is two-thirds of the natural size. The specimen before me, collected 

 by Mr. Brewer, is six and a half inches in diameter. 



Locality: North fork of Cottonwood Creek, Shasta County (Div. A.). The 

 specimens are all fragmentary casts in a sandstone, and are so distorted as to 

 render it impossible to ascertain the shape of a section of a whorl ; or even to 

 show the amount of enveloping of the whorls. 



The character of the ribs indicates that this species is related to A. Belknapii, 

 Marcou; also to A. Pcruvianus, Yon Buch, and remotely to A.Jiaccidicosta, Boemer. 

 From the first two, it can be distinguished by the dorsum being distinctty rounded 

 and not carinate; from the latter, by the more numerous and more distinctly 

 curved ribs and the abrupt umbilical margin. 



A. Haydexii, n. s. 



PL 10, Fig. 8, and 8 a, b. 



Shell compressed discoidal. Whorls about four, or more, 

 deeply embracing, flattened on the sides, nearly flat on the dor- 

 sum, abruptly truncated in the umbilicus. Surface smooth, or 

 marked by very faint undulations. 



