580 UNITED STATES GEOLOGICAL SURVEY OF THE TERRITORIES. 



narrowly rounded at the extremity; dorsal outline declining from the beaks 

 at an angle of about 90°, the anterior slope being more abrupt than the 

 other; beak moderately prominent, incurved, gibbous, and located in advance 

 of the middle; posterior umbonal slope very prominent, or subangular from 

 the beak obliquely backward and downward to the posterior basal extremity ; 

 surface marked by fine lines of growth, with a few rather distinct concentric 

 wrinkles below the middle. 



Length, 0.75 inch; height, 0.55 inch; convexity, 0.23 inch. 



The type-specimen of this species is a left valve; hut a few very imper- 

 fect right valves were found associated with it, that probably belong to the 

 same species. These, however, seem to present the unusual appearance of 

 beinsj less convex and thinner than the right valve. One of them shows the 

 tooth of the hinge to be moderately strong, obtuse, and curved upward. 



Compared with the last, this species will be seen to be readily distin- 

 guished by having its left valve decidedly more convex, its beak much less 

 pointed, and not directed so obliquely forward. If the right valves discov- 

 ered with it really belong to the same species, they also will be found to 

 differ materially, being much few convex than that of the last. 



It is, however, more nearly related to the form described by me under the 

 name Corbula tropidopkora, from one of the Upper Cretaceous sandstones . 

 near Point of Rocks, Wyoming, on the Union Pacific Railroad. The latter, 

 however, has its posterior umbonal slope much more distinctly angular, and 

 its beak less convex and not so incurved, while its surface has more distinct 

 concentric markings. These forms, however, are evidently closely-allied 

 species. 



Locality and position — Judith River group, or Brackish-water Lignite 

 series at the mouth of Judith River, Montana: probably belonging to the 

 latest division of the Cretaceous. 



Corbula p «■ r ii u «1 a t a , M. & H. 



Plate 40, figs. 4, a,o,c,&. , 



Corbula perundata, Meek ami Hayden (185(3), Proceed. Acad. Nat. Sci. Philad., VIII, 116. 



Shell small, longer than high, transversely ovate-subtrigonal, rather 

 compressed ; dorsal side sloping toward the extremities at an angle of about 

 80° from the beaks: anterior side rather abruptly rounded below; base 



