292 PALAEONTOLOGY. 



the 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, conc-entrlc lines of 

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

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

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



The shell dllfers tVoni ('. .^ e.rlciiuatus Meek and Hayden (Pal. Upper 

 :MissourI, p. 78, plate ill, ilg. G) In its more attenuated beak, flattened valve, 

 liner stria-, and more elongate form. 



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

 Ivawllngs Station, Wyoming. 



Genus LIMA Bnig. 

 Lima (Plagiostoma) occidentalis n. sp. 



ri.ato VII, li-. 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 postenor 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 tlie 

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

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

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

 ligamental area not obsen-ed. 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, aljout 

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

 Avith a few incipient radii observable on the anterior slope. Tlie radii are 



