Tlic Fauna of the Maxinllc Limcstoi 



369 



WOS. Dcrbya crassa. Girty, U. S. Geol. Surv., Prof. Paper 16, p. 347. 

 Hermosa formation; San Juan region, Ouray, Colorado. 

 Weber limestone and Maroon formation : Crested Butte district, 



Colorado. 

 Weber formation : Leadville district, Colorado. 

 1904. Derbya crassa. Girty, U. S. Geol. Surv., Prof. Paper 21, p. 52, 

 pi. 11, fig. 3. 

 Naco limestone : Bisbee ciuadrangle, Arizona. 

 1909. Orthothetes {Derbya) crassus. Grabau and Shimer, N. A. Index 

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

 Upper Carbonic: North America. 



Description. — "Shell very variable in size and form, but 

 usually more or less plano-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. Ventral valve more or less 

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

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



Fig. 7. — 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 

 abruptly and which is commonly distorted. 



moderate height with a covered deltidium; beak more or less 

 distorted. Dorsal valve convex, often quite rotund, but usually 

 depressed convex, with a slightly prominent umbo. Surface of 

 the shell marked by radiating striae of considerable strength, which 

 are sometimes sharply elevated and uniform, but on other speci- 

 mens may be distinctly alternating in strength or arranged in 



