BUIXETIN 31 30 



figs. 1-2. 



Conrad's original description. — Shell convex, with slight concentric 

 undulations ; umbo tapering gradually towards the apex, which is acute, 

 but not prominent ; wings large and very oblicjiie ; sinus of the posterior 

 margin not profound ; nacre very pearly and irridescent. Height i \( inches. 



This .species seems to var}^ considerably in obliquity, and 

 thickness of shell. It seems never to occur in perfect condition ; 

 generally only the thicker, iimbonal region is found. By exam- 

 ining the fragments, however, it becomes apparent that the wing 

 was quite pointed ; rudiments of cardinal teeth in left valve ob- 

 vious ; ligament submergent, broad. 



Type- Phila. Acad. Nat. Sci. 



Geological horizon. — Claibornian and St. Maurice, and per- 

 haps Sabine Eocene. None with the thickening of the valves 

 shown by the figures on pi. 16 (figs. 3-7) have come to the writ- 

 er's attention from horizons below the Claibornian. 



Specimens Jigm-ed. — Claiborne sands, Paleont. Mus. Cornell 

 Univ. 



A fairly perfect specimen of a left valve, thinner and much 

 more erect, possibly of this species, has been found 2 miles \V. of 

 Crockett, Houston Co., Tex. Coll. Univ. of Tex. 



Pinna gravida, n.sp., PI. 16. Fig. S-ii. 



Specific characterization. — Shell moderate in length, but of 

 extreme convexity, giving a cross-section half way from beak to 

 posterior almost a circular form ; nearer the beak the cross-sec- 

 tion appears to be elliptical, wider than high. Markings on 

 shell rarely of sufficient strength to show through on cast of inter- 

 ior (usually the only form in which the species is found). Rare- 

 ly on imprints of exterior a few radiating raised costae are shown 

 on the superior part of the shell, while commonly concentric un- 

 dulations are seen near the basal portion. (Compare form with 

 P. vexi///nn Born. Recent.) 



Type. — Deposited in Paleont. Mus. Cornell Univ. 



Horizon. — St. Maurice Eocene. 



Locality. — St. Maurice, La. Fragments of what appear to 

 be of this species are abundant in the St. Maurice of Texas. 



