196 PALAEONTOLOGY OF CALIFORNIA. 



This species cannot be confounded with C. truncata. It is thinner, more con- 

 vex, less oblique, less produced behind, and the ornamentation of the surface is 

 entirely different. 



C. TRUNCATA, n. S. 

 PI. 25, Fig. 182. 



Shell large, thick, subquadrate, obliquely truncate behind, 

 rounded in front, and varying from nearly straight to prominently 

 convex below; beaks moderate in size, very prominent, nearly 

 central, distant and inclined slightly forwards. Surface of young 

 specimens marked by pretty regular, fine, radiating lines, slightly 

 elevated, and sometimes exhibiting a tendency to alternation in 

 size ; these lines usually become nearly obsolete in large speci- 

 mens, and the lines of growth, which in the young shells are 

 barely visible, then form the most obvious character of the sur- 

 face; area broad, and marked by eight or ten angular lines. 

 Hinsre robust, broad at the ends and narrow in the middle; com- 

 posed of three or four long, angular teeth at each end, and a few 

 small, variable, transverse ones in the centre. Internal plate 

 robust, but not very elevated. 



Figure, average size. 



Localities: Abundant in Division A., at Curry's, south of Mount Diablo; found 

 also at Tuscan Springs, Tehama County ; Benicia; Martinez; Texas Flat, Placer 

 County, and at many points in the Coast Range, On the east side between Mount 

 Diablo and Pacheco's Pass ; also at Jacksonville, Oregon. 



This shell is allied to C. Nebrascensis, Owen ; but differs in the smaller beak, pro- 

 portionally longer form, more oblique truncation posteriorly, and in being more 

 produced in the anterior basal region. There is also a difference in the hinge. 

 In C. Nebrascensis the lateral teeth radiate, as it were, from an imaginary point ; 

 while in the present species they are parallel with each other and with the upper 

 edge of the hinge-plate, and their inner ends are bent at a right angle. 



For aid in the above comparisons I am indebted to Mr. F. B. Meek, who fur- 

 nished mi! with sketches of Dr. Owen's species, much more accurately drawn than 

 the published figures in Owen's Report, pi. 7, fig. 1. 



Except C. Maconensis, Con., this is the largest Cucullma found in America. A 

 specimen before me, from the south side of Mount Diablo, measures three inches 

 from beak to base ; and I have seen fragments which appeared to belong to even 

 larger shells 



