292 rAL^.ONTOLOGY. 



tlie beak to a little above the middle of the length, and embracing an angle 

 of about ninety degrees; anterior wing narrow, separated from the body of 

 the shell by a deep, sharply rounded notch, which extends about half-way 

 to the center of the valve; a strongly-depressed sinus passes from the base 

 of the notch to the beak, separating the upper part of the wing from the 

 body of the shell. 



Surface of the shell marked by rather distinct, concentric lines of 

 growth, which are crossed by exceedingly fine, radiating striae, imperceptible 

 except by the aid of a lens. The striae diverge rapidly from the median 

 line, and curve upward rather more strongly toward the sides of the shell. 



Tlie shell differs from C. f extenuatus Meek and Hayden (Pal. Upper 

 Missouri, p. 78, plate iii, fig. G) in its more attenuated beak, flattened valve, 

 finer strife, and more elongate form. 



Formation and locality. — In limestone of Jurassic age, northwest of 

 Rawlings Station, Wyoming. 



Genus LIMA Brug. 

 Lima (Plagiostoma) occidentalis n. sp. 



Plate VII, fig. 23. 



Shell of medium size or smaller, very broadly ovate, the height slightly 

 exceeding the greatest width; widest point about one-third of the height 

 from the basal extremity, below which the margin is regularly and evenly 

 rounded, and above the posterior border is more rapidly rounded and con- 

 tracted to the extremity of the short hinge-line, with which it blends with- 

 out perceptible angle. Valves strongly convex, most ventricose near the 

 middle of the anterior border, from which point the surface declines to the 

 postero-cardinal and postero-basal margins; beaks apparently small, incon- 

 spicuous, and somewhat appressed (?). Anterior wing minute, posterior 

 wing^mall, but alated, not distinctly separated from the body of the shell; 

 ligamental area not observed. Anterior border of the shell strongly con- 

 cave, the concavity extending more than half the height of the shell. 



Surface of the shell marked by simple, strong, rounded radii, about 

 sixty in number, posterior to the junction of the anterior and basal borders, 

 with a few incipient radii observable on the anterior slope. The radii are 



