PROCEEDINGS OF UNITED STATES NATIONAL MUSEUM. 297 



Kansas, where it was discovered by Prof. B. F. Mudge associated with 

 the preceding species, and also with Gyrena Dakotensis Meek & llayden 

 and Cardium ? kansasense Meek. 



Genus PACHYMYA Sowerby. 



Pachymya ? compacta (sp. nov.). Plate 6, figs. 3 and 4. 



Shell small, narrower posteriorly than anteriorly, slightly gaping be- 

 hind ; beaks depressed, approximate, incurved, directed forward, their 

 position being very near the front ; basal margin broadly convex ; pos- 

 terior margin narrowly rounded; postero-dorsal margin forming an 

 oblique downward and backward truncation of that part of the shell ; 

 cardinal margin nearly straight, subparallel with the basal margin, 

 much shorter than the full length of the shell ; ligament short, its area 

 depressed and sharply defined ; front very short, depressed beneath the 

 beaks and narrowly rounded below; umboual ridges prominent and an- 

 gular or subangular; the space above and behind them moderately 

 broad and flattened ; the remainder of each valve somewhat regularly 

 convex. Hinge and interior markings unknown. Surface marked by 

 the ordinary concentric lines of growth. 



Length, 29 millimeters ; height, 18 millimeters ; thickness, both valves 

 together, 14 millimeters. 



This species is e\ddeutly congeneric with the shell which in the An. 

 Eep. TJ. S. Geol. Sui\ Terr, for 1877, p. 298, I described as Pachymija f 

 herseyi, and also with the Cypricardia f texana of Eoemer, but knowing 

 nothing of the hinge of either of these forms, I am not satisfied that 

 they are j)roperly referable to Pachymya ; yet in all their external char- 

 acters they seem to agree. 



Position and locality. — Cretaceous strata, Bell County, Texas, where 

 it was collected by Mr. D. H. Walker. 



Genus THRACIA Leach. 



Thracia myaeformis (sp. uov.). Plate (i, lig8. 1 and "2. 



Shell transversely subov ate in marginal outline; valves nearly equal; 

 anterior end regularly rounded; wider and thicker anteriorly than 

 posteriorly ; posterior portion narrowed vertically and somewhat com- 

 pressed but gaping at the extremity; basal border broadly convex; 

 posterior border abruptly rounded; cardinal margin slightly convex, 

 but the prominent umbones give the shell a concave appearance behind 

 the beaks ; a distinct linear depression is seen in the natural cast upon 

 each side of the ligament; beaks prominent, incurved and directed a 

 little forward; muscular impressions not distinctly shown in our ex- 

 amples, which are natural casts in chalky limestone, but the pallial 

 sinus appears to have been large and subangular at its anterior end. 

 Surface marked by the ordinary lines of growth, and also by more or 

 less distinct irregular concentric wrinkles. 



