280 REPORT UNITED STATES GEOLOGICAL SURVEY. 



larly rounded ; postero-dorsal border nearly straight from the posterior 

 border to the base of the posterior wing ; beaks having the ordinary 

 prominence 5 surface smooth or having the usual concentric lines of 

 growth. 



Length, from the end of the anterior wing to the posterior extremity 

 of the shell, 31 millimeters ; breadth across the widest part of the body, 

 15 millimeters. 



This species resembles the Avicula lingulifera of Shumard, but it dif- 

 fers from that species in its more elongate form and more oblique hinge- 

 line. 



Position and locality. — Cretaceous strata, probably equivalent with 

 those of the Fox Hills Group ; south of Grand Eiver, Middle Park, 

 Colorado, where it was collected by Prof. J. W. Powell. 



Subgenus Oxytoma Meek. 

 Pteria (Oxytoma ?) gastrodes. 



Plate x, fig. 1 a. 



Avicula (Oxytomal) gastrodes Meek, 1873, An. Rep. U. S. Geol. Surv. Terr, for 1872, p. 

 491. 



" Shell (as determined from a left valve) attaining a moderately large 

 size, subtrigonal in outline, rather distinctly convex, and having a very 

 slight backward obliquity ; basal outline very profoundly rounded, the 

 deepest or most prominent part being in advance of the middle ; poste- 

 rior margin moderately sinuous below the wing, from the extremity of 

 which it ranges obliquely forward and downward, rounding regularly 

 into the base below ; anterior margin strongly and subangularly sinuous 

 under the wing, thence descending with a slight forward obliquity and 

 rounding rather abruptly into the base; hinge-margin longer than the 

 height of the valve, the anteroposterior diameter of which (at any point 

 below) it also decidedly exceeds, ranging nearly at right-angles to the 

 vertical axis of the shell ; beak distinctly convex, rising above the hinge- 

 margin, strongly incurved, without obliquity, and situated less than one- 

 third the length of the hinge-margin from the extremity of the anterior 

 wing, which is subtrigonal in form, somewhat convex, a little rounded at 

 the extremity, and very strongly separated from the abrupt shell [swell?] 

 of the umbo by a deep rounded concavity extending from the beak ob- 

 liquely to the marginal sinus below; posterior wing longer and more 

 compressed, narrower and more angular than the other ; both wings, 

 particularly the posterior one, projecting decidedly beyond the margin of 

 the valve below. Surface only showing more or less distinct lines of 

 growth, which are stronger on the wings than elsewhere. Eight valve 

 unknown. 



"Height of left valve, 1.50 inches ; length of same below the wings, 

 about 1.30 inches ; length of hinge-hne, 1.90 inches ; convexity (of left 

 valve alone), 0.40 inch. 



"Locality and position. — Cretaceous sandstones, Coalville, Utah." 



Only a single example of this species has yet been discovered, although 

 I have carefully searched the locality since Mr. Meek originally discov- 

 ered it. It is in the condition of a sandstone cast, but its external char- 

 acters are quite clearly shown. It is well represented by fig. 1 a, plate 

 x, except that the great convexity of the body of the valve does not fully 

 appear. Mr. Meek adds the following remarks to his original descrip- 

 tion of this species : 



