PEOCEEDINGS OF UNITED STATES NATIONAL MUSEUM. 293 



MOLLUSCA. 



Genus OSTEEA Liuuaeius. 



Subgenus Alectryonia Fischer. 



Ostrea (Alectryonia) blackii (sp. nov.). Plate 4, figs. 1 and 2. 



Shell irregularly subovate in marginal outline, moderatelj'^ capacious, 

 beaks small, sometimes obscure and sometimes moderately prominent. 

 Lower valve usually moderately deep and capacious, its convexity being 

 more prominent about the middle than elsewhere, often snbalate, but 

 this latter feature is usually obscure ; scar of attachment at the beak 

 usually present and often moderately large; ligament-area usually short 

 and rather small, but sometimes comparatively large and laterally ex- 

 tended; ligament-furrow well defined and having the usual (;haracter- 

 istics of the genus. Upper valve nearly flat and corresponding with the 

 lower in other respects, except that it is not so broad along the hinge-bor- 

 der and never has there the subalations which sometimes mark the lower 

 valve. The a<lductor scars are moderately large and have the form com- 

 mon to Alectryonia, namely, curved-spatnlate. Surface of both valves 

 marked by concentric lines and strong imbrications of growth, and each 

 by a dozen or more radiating ribs or plications, which constitute a con- 

 spicuous feature of the shell ; but they are usually somewhat less dis- 

 tinct upon the upper than upon the under valve. 



Length, 68 u 

 32 millimeters. 



In form and general aspect this shell approaches that of a typical 

 Ostrea; but in the character of its adductor scars, the extent of its plica- 

 tions, and the subalation of its cardinal border it is properly referable 

 to Alectryonia. The only shell with which it need be compared is 0. 

 hellaplicata Shumard, also from Texas. It differs from that shell in 

 being constantly larger, proportionally less capacious, broader toward 

 the base, and in having its hinge-border longer and more oblique. 



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

 it was collected by Mr. S. W. Black, and sent by him to the Smith- 

 sonian Institution. The specific name is given in his honor. 



Genus EXOGYEA Say. 



Exogyra forniculata (sp. uov.). Plate 4, figs. 3 and 4. 



Shell subtrihedral in lateral outline, somewhat compressed vertically. 

 Under, or left valve thick, especially its umbonal half; beak curved 

 strongly toward the posterior border, and in the plane of the free mar- 

 gins of the valve, not forming so much as one complete volution, its 

 point being free but closely approaching the posterior border of the 

 valve; ligament-area irregularly triangular, moderately large, extend- 

 ing to the apex of the beak, its sulcus well developed; interior surface 



