CRETACEOUS PERIOD. 175 



never distorts the beak or impairs its symmetry. Smaller valve nearly flat 

 or slightly concave, suborbicular in outline. Sm'face of the smaller valve 

 marked by numerous thin imbricating striae of growth ; that of the larger 

 valve marked by numerous small radiating costae, which are quite distinct 

 upon the umbonal region, but become obsolete or absent upon the basal 

 portion of the valve. These costse are slightly crenulate or iiTegular, 

 increase a little in size from the umbo toward the base, increase in number 

 both by intercalation and bifurcation, and occupy only the external layers 

 of the test, exfoliated surfaces being free from them. 



Diameter, from umbo to basal margin, of the largest example in the 

 collection, forty-two millimeters ; transverse diameter, thu-ty-four milli- 

 meters ; convexity of the larger valve, nineteen millimeters. 



This shell reaches a size nearly equal to that of JE. Icevmscula Roemer ; 

 but it is distinguished from that species by the costate surface, less con- 

 vexity, and smaller umbo of the larger valve. 



From the typical fonns of JE. costata Say, as they are identified from 

 the Cretaceous strata of New Jersey, Alabama, and Texas, it differs in its 

 much smaller size, its tliinner and more delicate test, and proportionally 

 smaller costse. The collections contain numerous examples of this variety, 

 but none of the typical forms of E. costata. Some of the examples are very 

 young, but the usual uniformity of size prevails among the larger ones that 

 indicates adult age, and they have also the aspect of maturity. If it were 

 not for the well-known tendency to extreme variation among the Ostreid(S, 

 I should not hesitate to separate this as a distinct species rather than as 

 a variety. It is not improbable that, when full and careful comparisons 

 shall have been made, our shell will be found specifically distinct from E. 

 costata. In such a case, I propose the varietal name given above for a 

 specific one. 



Position and localitfi. — Strata of the Cretaceous period ; east bank of the 

 Rio Puerco, six miles below Casa Salazan, New Mexico. 



