llic Fauna of l/ic Ma.vvilU' Limestone. 



369 



1903. Dcrhya crassa. Girty, U. S. Geol. Surv., Prof. Paper 10, p. 347. 

 Hermosa formation : San Juan region, Ouray, Colorado. 

 Weber limestone and Maroon formation : Crested Butte district, 



Colorado. 

 \\'e-l)er formation: Leadville district, Colorado. 

 lliUl. Dcrbya crassa. Girty, U. S. Geol. Surv., Prof. Paper "21, p. 52, 

 pi. 11, fig. 3. 

 Xaco limestone : P)islxe quadrangle, x\rizona. 

 liliiM. Orflinlhctcs (Dcrbya) crassns. Grabau and Shimer, N. A. Index 

 Foss., p. 231, fig. 282 a-d. 

 Upper Carbonic : North America. 



Dcscrif'fioiL — "Shell very variable in .size and form, but 

 usnallv more or less ])lano-convex as seen in profile, somewhat 

 semi-oval in outline, but usually a little too long from beak to 

 base to be strictly so considered. A'entral valve more or less 

 flattened, a little prominent on the umbo, but tisually becoming 

 slightly concave toward the front of the shell ; cardinal area of 



', — Derbya crassa. 



a and b. — Impression of a ventral and dorsal valve. (After Whit- 

 field. ) Shell decidedly variable, but the ventral valve of 

 many specimens is very flat except for the beak which rises 

 aljruptly and which is commonly distorted. 



moderate height with a covered deltidium ; beak more or less 

 <listorted. Dorsal valve convex, often quite rotund, but usually 

 <lepressed convex, with a slightly prominent umbo. Surface of 

 the shell marked by radiating strise of considerable strength, wliich 

 are sometimes sharj)ly elevated and uniform, but on other speci- 

 mens may be distinctly alternating in strength or arranged in 



