CRETACEOUS FOSSILS. 77 



broad, and of almost the same pattern as the superior lateral, 

 being slightly more complex. 



Figure, four-fifths of natural size. 



This species is very closely allied to C. Duvalii, Lev. ; but there are several 

 strong points of difference. The section of the whorls, in tho present species, is 

 nearly twice as broad as in the figure given by D'Orbigny, Pal. Fr., vol. 1, pi. 

 113, or that by Bayle and Coquand, Mem. Gcol. Soc. France, 2 ser., vol. 4, pi. 3. 

 The whorls are smaller in diameter when young, and increase more rapidly in 

 size than in the figure given by D'Orbigny. The spaces between the whorls are 

 also larger in proportion to the size of the shells, and the number of large ribs to 

 8 volution is nearly twice as great as given by D'Orbigny. There arc also several 

 points of difference in the septum. The lobes are all more slender, and the inferior 

 lateral lobe is much smaller in proportion than in D'Orbigny's figure. The ven- 

 tral lobe, however, shows the greatest difference. In C. Duvalii it is small; in this 

 species it is as long as the superior lateral and more complex. 



Locality: Near "Weavervillc, in the Trinity Itivcr, Trinity County ; from a 

 boulder. 



C. PERCOSTATUS, 11. S. 

 PI. 1G, Fig. 2G; and PL 17, Fig. 2G a. 



Shell large; section of whorls rounded, subquadrate, as broad 

 as high, somewhat narrowed towards the dorsum ; sides flattened; 

 back flat, or but slightly convex ; ventral surface usually a little 

 concave, sometimes plain. Surface marked by plain ribs, small 

 and numerous on the younger whorls, but which become fewer 

 and larger as the shell increases in size, until in the adult state 

 they are large, prominent, isolated by broad interspaces, and some- 

 times forming a sixth of the whole height of the whorl. These 

 ribs commence at the ventral margin, curve backwards slightly, 

 and then pass directly to and across the back; they arc often 

 dichotomous in the young shells, but never so in the adult. 



Interspace between the volutions very small, being scarcely 

 greater than the thickness of the shell substance. 



Septum almost exactly like the succeeding. I have not been 

 able to see all of the parts, but such portions as I have been able 

 to study do not seem to differ in any details; although probably 

 perfect specimens would show some points of difference. 



