wHrrE.l PALEONTOLOGY — CRETACEOUS FOSSILS. 277 



115°. Inferior valve convex, most prominent along the middle, and 

 sometimes obtnsely snbangiilated ; beak acute, prolonged, situated near 

 the buccal side, and slightly curved towards the opi)Osite side ; muscular 

 impressions large, moderately excavated, elongate-oi^ate, upper edge 

 concave. Siuface of valves marked with i)rominent, concentric, waved, 

 imbricating lamiuse of gTowth and form, and from ten to fourteen ele- 

 vated, obtusely angiilated costce, which originate near the beaks and 

 radiate to tlie margms. 



" In many of the specimens before me all the ribs are simple, but in 

 others some of them are biturcated. In a few individuals they are 

 shari^ly angulated at their extremities. The concentric laminae are gen- 

 erally more distinct and more strongly marked on the superior than on 

 the inferior valve. 



"The dimensions of an average specimen are: Length, l^^g^ inches; 

 width, fi'om beak to base, 2f^ inches ; thickness, ^g inch. 



"This handsome oyster occurs in the greatest abundance in fine- 

 grained sandstone and blue indurated marl, towards the toj) of the Lower 

 Cretaceous, near Sherman, in the bluffs of Post Oak Creek, and various 

 other localities in Grayson County. It is found in connection with the 

 remains of Squalidcc^ Ostrea congesta, and Corbula graysonensis.^^ 



The figiu'es of this species on plate 8 are from j)hotograph copies of 

 Dr. Shumard's drawings. Those on plate 4 are drawn b3^ Mr. Holmes 

 from an examjjle collected by Professor Mudge at Denison, Tex. 



OsTEEA (Alectkyonla.) sannionis White. 



Plate 2, figs. 2 a, b, c, d, and e. 

 Ostrea sannhnts White, 1876, Powell's Eep. GeoL Uinta Mts., p. 112. 



Shell rather small, alate at both sides of the beak, irregularly snbquad- 

 rate in marginal outline, its longitudinal axis curved, the convexity of the 

 ciu'ved side being forward ; nearly or quite as wide across the alations 

 as at the base, but much constricted in the middle, especially upon the 

 posterior side ; beaks small, not prominent, directed a little backward. 

 Lower vahe moderately convex ; scar of attachment at the beak small 

 or absent; ligament-area short, rather broad, its longitudinal fiuTow 

 shallow, but weU defined, transversely striated and pointing obliquely 

 backward ; posterior alation narrower and more clearly defined than the 

 anterior one and a little longer than the corresponding alation of the 

 opiK)site valve ; muscidar scar comparatively large, situated nearly mid- 

 length of the valve and near the posterior margin, curved-spatulate in 

 outline, its broader end being toward the base of the shell and the con- 

 vexity of the curve toward the postero-basal jiortion. Upper valve 

 nearly flat, but its lateral and basal portions are made very irregular by 

 the coarse and deep pUcations w hich are common to both valves. 



Surface of both valves marked by the ordinary lines and laminations 

 of growth common to the genus, and also by somewhat nmnerous crenu- 

 lated radiating plications in the rostral region, the gTeater part of which 

 terminate before reaching the bordefs, but four or five of them, increas- 

 ing greatly in size as they proceed, reach the basal border of the shell, 

 to w^hich they give a strong zigzag character. 



Length from base to beak of one of the larger examples, 39 millime- 

 ters ; breadth near the li'ont of the same, also 39 milUmeters ; across the 

 alations, 33 millimeters. 



The species of the subgenus Alectryonia Fischer are, as a rule, less 

 subject to extreme variation than those of t^^jical Ostrea; and this is a 



