294 PROCEEDINGS OF UNITED STATES NATIONAL MUSEUM. 



having the usual characteristics of the genus. A faint, illy-definecl sul- 

 cus is apparent on the posterior side, extending from the umbo to the 

 basal border, between which sulcus and the laterally flattened-concave 

 posterior border of the va.lre there is an equally indefined radiating 

 curved ridge. The anterior portion of the valve is marked by a strong 

 angular, rough carina or rid.q;^ which extends from the beak to the basal 

 border. The prominence of this ridge gives a flattened aspect to the 

 outer surface of the valve, and also produces a flattened space of con- 

 siderable width between it and the anterior margin. Surface marked 

 by the ordinary coarse lines of growth, often presenting the coarse 

 imbrications so common to the Ostreidw; and upon the ridges just 

 described there are occasional nodes or vaulted projections of portions 

 of the shell. Upper valve unknown. 



Length, 70 millimeters; breadth across near the base, where it is 

 broadest, 50 millimeters. 



In general aspect this shell is much like a Gryphwa, but it is referred 

 to Exogyra because of its laterally instead of perpendicularly curved 

 umbo and beak. This species therefore affords additional evidence of 

 the well-known fact that the two genera named approach each other very 

 closely. Specifically this shell is well marked by its strong, rough angu- 

 lar carina, its free beak, narrow umboual region and broad base. la 

 these respects it differs too much from any described form to need de- 

 tailed comparison. By casual observation it may be mistaken for the 

 variety navia Conrad, of Grypliiea pltcheri ; but a comparison of the 

 beaks of the two forms will show a well-marked difference. 



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

 it was collected by G. W. Marnoch, esq., together with many well-known 

 Cretaceous species of that region. 



Exogyra vT-inchelli (sp. uov.)- Plate 2, figs. 1 uud 2; ami plate 3, figs. 1 aud :2. 



Shell of medium size, irregularly subovate in marginal outline; ses- 

 sile, or attached by a large part of the surface of the lower or left valve, 

 being oblicjuely inclined so that the anterior border is very much higher 

 than the posterior. Lower valve massive, moderately deep, its front 

 side nearly perpendicular and of considerable height vertically; umbo 

 vertically flattened continuously with the front side, and broadly curving 

 backward ; beak closely incurved under the posterior border and there 

 concealed; ligamental groove long and narrow, occupying the whole 

 curvatiu-e of the umbo. Upper valve nearly flat, thick; the anterior 

 part being much thicker than the posterior; beak vertically thin or 

 compressed, closely coded in a plane with that of the valve, making a 

 little more than one entire volution. Surface marked by coarse lines of 

 growth, and near the anterior borders of both valves, especially the 

 upper, it is usually deeply laciniate. 



Length, 90 millimeters; breadth, G6 millimeters: height in front, 55 

 millimeters. 



This species belongs to the same section of the genus Exogyra with 



