INVERTEBRATE PALJSONTOLOi i V 519 



are sometimes presented by different varieties of a species among the 

 existing Uniones, with the materials yet available for comparison, I do not 

 feel warranted in uniting these two forms under one specific name. 



Locality unci position. — Judith River group; mouth of Judith River, 

 Montana. Probably belonging to the closing part of the Cretaceous epoch. 



V 11 i O I » <■ w «• > :i I* II % . M & H 



Plate 41, figs. 2, a, b, c. 

 Cut, i Deiveyanw, Meek and Hayden (1857), Proceed. Acad. Nat. Sci. Philad., IX, 145. 



Shell very thick, transversely narrow-subovate or subelliptical, rather 

 convex in the anterior region; extremities rounded; base semi-elliptic, or 

 nearly straight along the middle, and rounding up to the extremities ; dorsal 

 margin straight, or declining very gradually from the beaks posteriorly ; beaks 

 small, depressed, located near the anterior end ; surface (the epidermis and 

 outer fibrous layer being mainly removed) having obscure marks of growth, 

 crossed by very fine, indistinct, radiating striae; ligament long; cardinal 

 teeth very thick and strong; lateral teeth long, slightly arched; muscular 

 cicatrices rather deep and strongly corrugated; concavity of the beaks 

 shallow. 



Length, about 2.60 inches; height, 1.33 inches; breadth, about 1.18 

 inches. 



The specimens yet obtained being all worn, and more or less mutilated, 

 do not show very clearly the form of the adult shell ; but the curves of the 

 marks of growth exhibit the outline of young and medium-sized individuals 

 quite satisfactorily. In some instances, the interior still retains a peculiar 

 tinge, indicating that the nacre was originally purple or flesh-colored. 



Although somewhat related to the last, this species differs in being much 

 thicker and more gibbous in the umbonal region, less elongated, and in having 

 the basal border more convex in outline. Its dorsal margin is also straighter, 

 while its beaks are removed a little farther from the anterior extremity. As 

 suggested by Mr. Lea, it is evidently similar in form and obliqueness, as well 

 as in its rather unusual thickness, to the recent U. truncatus, Swainson, 

 although not more than half the size which that species generally attains. 

 It was named in honor of our esteemed friend, Prof. Chester Dewey, 

 deceased, of Rochester University, Rochester, New York, 



Locality awl position. — Same as last. 



