white.] PALEONTOLOGY — CRETACEOUS FOSSILS. 277 



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

 sometimes obtusely subangulated ; beak acute, prolonged, situated near 

 the buccal side, and slightly curved towards the opposite side ; muscular 

 impressions large, moderately excavated, elongate-ovate, upper edge 

 concave. Surface of valves marked with prominent, concentric, waved, 

 imbricating laminse of growth and form, and from ten to fourteen ele- 

 vated, obtusely angulated costse, which originate near the beaks and 

 radiate to the margins. 



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

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

 sharply angulated at their extremities. The concentric laminse 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, 1^ inches ; 

 width, from beak to base, 2 t 3 q inches ; thickness, t 8 q inch. 



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

 grained sandstone and blue indurated marl, towards the top 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 Squalidce, Ostrea congesta, and Corbula graysonensis. n 



The figures of this species on plate 8 are from photograph copies of 

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

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



Ostrea (Alectryonia) sannionis White. 



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

 Ostrea sannionis White, 1876, Powell's Rep. Geol. Uinta Mts., p. 112. 



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

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

 curved 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 valve moderately convex ; scar of attachment at the beak small 

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

 shallow, but well 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 

 opposite valve ; muscular 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 portion. Upper valve 

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

 the coarse and deep plications which 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 numerous crenu- 

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

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

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

 to which they give a strong zigzag character. 



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

 ters ; breadth near the front of the same, also 39 millimeters ; across the 

 alations, 33 millimeters. 



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

 subject to extreme variation than those of typical Ostrea; and this is a 



