CRETACEOUS PERIOD. 181 



upward from the base and rounded to the cardinal margin ; hinge-lme long, 

 sometimes nearly parallel with the basal border, and never much divergent 

 from it. 



Surface marked by moderately strong folds or undulations, and also by 

 fine lines of growth. 



Long diameter, of an average-sized example, about seven centimeters ; 

 shorter diameter, at right angles with the former measurement, four and a 

 half centimeters. 



The conspicuous features of this species are its long, slightly divergent 

 hinge-line, and its inflated truncated front. 



It is a fact worthy of note that the greater part of the examples of this 

 sjDecies in the collections have the pearly layer of the test in a better state 

 of preservation than the prismatic layei', while in the numerous other species 

 of the genus that have come under my observation, the pearly layer is 

 usually, if not always, wanting. 



Position and locality. — Strata of the Cretaceous jDcriod ; one and a quarter 

 miles south of Bowlder, Colorado, and at Callinas Creek, New Mexico. 



Inoceramus dimidius White. 

 Plate XVI, fig. 2 a, b, c, aud d. 



Inoceramus dimidins White, 1874, Expl. & Snrv. west 100th Merid., Prelim. Rep. Invert. 



Foss., 25. 



Shell very small for one of this genus, inflated, sometimes much so, 

 obliquely subovate in outline ; valves subequal, the left one being a very 

 little more capacious than the other ; test thin ; beaks small, prominent, 

 acute, incurving', and pointing a very little forward ; hinge-line straight or 

 nearly so, rather short. 



Surface marked by more or less regular and more or less sti-ong con- 

 centric folds or undulations. In some cases, these undulations continued to 

 be formed only until the shell had attained about half its full size, when they 

 ceased, the remainder of the sm-face being marked only by ordinary con- 

 centric lines of growth. This irregularity in the formation of concentiic 

 folds is sometimes connected with considerable distortion of the usual 

 symmetry of the shell. 



The long diameter of an average example from the umbo to the postero- 



