white.] PALEONTOLOGY CRETACEOUS FOSSILS. 287 



led by the imperfection of the specimens then obtained as to the true 

 character of the hinge. Further collections made at the typical locality 

 by myself have enabled me to correct the description as above and as 

 shown in fig. 2 &, plate 6, which is in part a restoration. No entirely per- 

 fect example of the hinge has yet been discovered, the posterior terminal 

 portion of series shown in the figure having been reproduced from 

 another example, and the exact condition of the middle portion of the 

 series is not positively known, the doubt in this respect being expressed 

 by the imperfection in the drawing at that point. 



Position and locality. — From strata apparently equivalent with those 

 of near the top of the Colorado Group, or the base of the Fox Hills Group ; 

 Coalville, Utah. At this locality, the division between those two groups 

 is not easily determined. 



Genus CEASSATELLA Lamarck. 



Crassatella cemarronensis (sp. nov). 



Plate 5, figs. 3 a, h, and c. 



Shell suboval or subtetrahedral in outline by lateral views, moderately 

 gibbous ; umbonal ridge usually well defined ; valves regularly convex 

 in front of it and flattened and a little compressed behind it; dorsum mod- 

 erately long and nearly straight as seen by side view ; front obliquely 

 truncated to about midheight of the shell, from which point it is regu- 

 larly rounded to the base ; basal border broadly and regularly convex ; 

 posterior border obliquely truncated from the dorsum to the postero-basal 

 border, which is abruptly rounded to the base; lunule narrow, well de- 

 fined, moderately deep, short-lanceolate or narrow-oval in outline; escutch- 

 eon comparatively long, lanceolate in outline, well defined by seemingly 

 raised lateral boundaries ; beaks small, situated well toward the front, 

 approximate, scarcely incurved, directed, but not deflected toward the 

 front; hinge-plate strong; cardinal teeth well developed, except the 

 posterior tooth of the right valve, which seems to be rudimentary ; liga- 

 mental fosset large ; muscular and pallial impressions well marked and 

 characteristic ; free border of the valves distinctly crenulated. Surface 

 marked by fine irregular concentric strise of growth and also by more 

 or less distinct concentric ridges and furrows. These characters are not 

 well shown in the illustrations of this species on plate 5, because the 

 specimen selected for figuring was covered by an incrusting film of lime- 

 carbonate, as were most of the examples collected. Wherever this 

 incrustation is removed, however, the concentric furrows and ridges are 

 usually quite distinct. 



Length, 32 millimeters; height, 25 millimeters; thickness, 18 milli- 

 meters. 



This species presents a good degree of variation as shown among the 

 specimens in the collection, some being more nearly subcircular and more 

 gibbous than the one figured on plate 5 ; in which respect they some- 

 what resemble C. evansi Hall & Meek, but the typical examples of the 

 species depart too widely from that species to need comparison. In 

 most of its characteristics it also agrees well with those of the subgenus 

 Pochythwrus of Conrad, but seems to differ from that form in the rudi- 

 mentary condition of its posterior cardinal tooth of the right valve 

 before mentioned, and also in the incipient development of a posterior 

 lateral tooth in the right valve and an anterior one in the left valve, 

 both of which extend below the lower border of the hinge-plate. 



