518 UNITED STATES GEOLOGICAL SURVEY OF THE TERRITORIES. 



This species will be readily distinguished from the last by its thicker, 

 more depressed shell, more attenuated posterior and sinuous base and flanks; 

 and probably also by having smooth instead of delicately-wrinkled beaks. 

 Its specific name was given in honor of the distinguished scientist, Prof. 

 James D. Dana, of New Haven. 



Locality and position. — Fresh- and Brackish-water Lignite series of the 

 Judith River. Among the collections from near the mouth of Yellowstone 

 River, there are also some larger internal casts of a Unio agreeing in general 

 form, arcuate base, and sinuous flanks with this. The impressions of the 

 anterior hinge-teeth and muscular cicatrices in these specimens are also very 

 strongly defined, and show that the former are deeply corrugated and the 

 latter quite strong. The posterior hinge-teeth are also seen to be very long. 

 The largest of these specimens measure nearly 5 40 inches in length, and, 

 making allowance for the indicated thickness of the sbell in the umbonal 

 region, must have been 2.20 inches in height, and nearly the same in con- 

 vexity. These, I am inclined to believe, belong to the species here described ; 

 but good specimens of the shell itself may show them to be entirely distinct; 

 if so, they might be called Unio permusculoms. 



U ■■ i o s ii b s p a t ii I a t ii s , M. & H. 



Plate 41, figs. 1, «, h. 

 Unio suispatulatus, Meek and Hayden (1857), Proceed Acad. Nat. Sci. Philad., IX, 146. 



Shell transversely elongate-snbovate, or nearly subspatulate in outline, 

 compressed, most convex near the anterior end, cuneate behind ; substance 

 rather thick ; anterior side subtruncated, but rounding into the base below ; 

 base most convex in outline anteriorly, and nearly straight or slightly sinuous 

 farther back ; posterior side very long, and narrowly rounded at the extrem- 

 ity ; dorsal outline slightly arcuate, nearly parallel to the base, or declining 

 with a very gentle curve behind; beaks small, oblique, rising little above the 

 dorsal margin, and located at the anterior extremity ; surface (the epidermis 

 and outer fibrous layer being destroyed) having rather obscure marks of 

 growth, crossed by indistinct radiating striae; binge and interior unknown. 



Length, 2.83 inches, height, 1.34 inches; breadth, 0.75 inch. 



It is barely possible that this may be a variety of the last, though it 

 differs decidedly in having its beaks more prominent and almost terminal, as 

 well as in being more compressed. Although as great differences of form 



