172 ■ CRETACEOUS PERIOD. 



and incurved, or flattened and short ; scar of attachment small or wanting ; 

 surface sometimes distinctly lamellose, but generally somewhat smooth, 

 although marked by concentric lines of growth. 



Upper valve nearly flat, moderately thick in the umbonal region ; 

 hinge-line well defined, straight ; area distinct ; ligamental groove small ; 

 inner surface smooth, more or less distinctly crenulated at the lateral edges ; 

 outer surface marked by numerous concentric, imbricating lines of growth, 

 and sometimes also by faint, impressed, radiating- stri«. 



The collections contain numerous examples of this widely-known 

 'species, none of which, however, are of so large a size as are some of those 

 figured by Roemer, Conrad, and others. In selecting examples for illustra,- 

 tion, I have chosen representatives of two extremes of fonn fi'om among 

 others of all intermediate gradations. Mr. Conrad states (loc. cit.) that there 

 are two distinct varietal types of this species, one of which resembles 

 G. vesiadaris Lamarck, and which was the typical form described and 

 figured by Dr. Morton ; and the other he designates as var. navia. The 

 collections under examination, however, although they contain representa- 

 tives of the two fomis referred to by Mr. Conrad, seem to indicate no con- 

 stancy of separate varietal characters, either of those forais or any others. 



The largest specimens in the collections have a length of only about 

 thirty-seven millimeters from the umbo to the basal margin, which is con- 

 siderably less than that of some examples reported by other authors. 



Position and locality. — Strata of the Cretaceous period : — twenty -five 



miles southeast of, and also near, Paria ; East Fork of Paria Creek ; east of 



Impracticable Ridge ; Dirty Devil Eiver, east of ridge, Utah : Canon 



five miles west of Puerco ; east of Moimt Taylor ; and Acoma Plateau, New 



Mexico, &c. 



Genus EXOGYRA Say, 1819. 



Exogyra ponderosa Rcenier. 



Plate XIV, fig. 1 a, b, and c. 



Exogyra ponderona Eoemer, 1852, Kreidebildung von Texas, 71. 

 Exogyra costata Courad, 1857, U. S. & Mex. Bound. Surv., i, 154. 



Shell large, capacious ; marginal outline irregularly subovate ; larger 

 valve very gibbous ; umbo distinctly spiral, but the coil is usually obsciu-ed 



