CRETACEOUS FOSSILS. 179 



Length, 1.7 inch ; width, 1.2 inch ; thickness, .75 inch. 



From Cottonwood Creek, prohably from the Shasta Group. A number of speci- 

 mens were collected by Mr. Mathewsonin 1865, from a dark-colored rock, in which 

 he does not seem to have found any other fossil; and beyond the meagre label, 

 "Cottonwood," I have no information. 



ARCOMYA, Agas. 



A. UNDULATA, 11. S. 

 PI. 29, Fig. 67. 



Shell very inequilateral, beaks prominent, strongly incurved, 

 and placed about a third of the length from the anterior end ; 

 cardinal line nearly straight; base slightly convex; ends rounded, 

 subtrnncated, and nearly equal; both ends slightly gaping-. Sur- 

 face unknown, surface of the cast marked by a few large, con- 

 centric undulations. 



Length, 2.25 inches; width, 1.3 inch ; transverse diameter, .9 inch. 



A single specimen, from Indian Creek, Butte County, belonging to Mr. Voy, of 

 Oakland, who kindly loaned it to me for description. 



Easily distinguished by its peculiar arcaeform shape, from any other known 

 fossil of California. 



HOMOMYA, Agas. 

 H. coNCENTRicA, Gabb. 



(Panopceaid., Gabb; Pal. Cal., Vol. 1, p. 148, pi. 22, fig. 119.) 

 From the Chico Group, of Tuscan Springs, Cottonwood Creek, and Martinez. 



MACTRA, Linn. 



? M. TENUISSIMA, U. S. 

 PL 29, Fig. 68. 



Shell small, extremely thin, subequilateral ; beaks central, 

 small, pointed slightly in advance; anterior end broadly and 

 regularly rounded ; posterior end obliquely and convexly sub- 



