EEPORT ON PALyEONTOLOGICAL COLLECTIONS. l)^~) 



cated above, and more narrowly rounded below, than the other: basal border semi- 

 elliptical in outline; dorsal side nearly straight along the middle: beaks very small, 

 depressed nearly to a level with the dorsal margin, not eroded, and apparently without 

 wrinkles, located about half-way between the middle and the anterior end: posterior 

 umbonal slopes rather prominently rounded: surface smooth, or only showing obscure 

 marks of growth. 



Length, 1.65 inches; height, 1 inch: convexity, 0.60 inch. 



The specimens of this species in the collection are not in a condition to show the 

 hinge, though some casts of the interior retain impressions of the lateral teeth, which 

 are comparatively long and straight. These casts also show the muscular impressions 

 to be moderately deep, and the cavity of the beaks rather shallow. 



In size and form, this species resembles Vnio nuralis, Mock and Ilayden, from near 

 the Black Hills, Nebraska; but its beaks are less elevated, and not so* gibbous; they 

 also appear never to possess the small concentric wrinkles characterizing those of that 

 species; and it seems likewise to be a. thinner shell than C. uhcuUs. Some varieties of 

 it resemble Mya teUinoides, Hall ( Fremont's Kept., ;>n7, plat*' .">. tig. 1 ), which is doubt- 

 less also a Unio; but they always differ from the figure cited in haying less elevated 

 beaks, and in being proportionally broader posteriorly. Named in honor of Dr. F. V. 

 Hayden, who has brought man v specimens of the species from the Far West. It seems 

 to come from a formation that Dr. Ilayden has called the Bridger group. 



Locality awl position.— Fresh-water Tertiary beds, near Fort Hridger, and south of 

 there, at the base of Uintah Mountains, latitude^ 40' north, longitude 1 10° W west. 



Genus GONIOBASIS, Lea. 



Mvhniia SimpMiri, Meel< (July, lsfiii), Proceed. Acad. Xat. Sei. Philad.. XII, 313. 



Shell elongate-conical: spire attenuated and pointed; volutions about ten, flattened 

 or more or less convex, increasing gradually in size, last one rounded below: suture 

 sometimes linear, in other instances more strongly defined, in consequence of the 

 greater convexity of the whorls; surface marked by fine lines of growth, and small. 

 slightly-arched, vertical folds, which vary in size and regularity on different specimens, 

 and are crossed by small, obscure, thread-like revolving -lines: aperture ovate; colu- 

 mella moderately sinuous below; lip somewhat retreating above, and prominent below 

 the middle. 



Length, 0.78 inch; breadth, 0.30 inch; apical angle nearly or quite regular, diver- 

 gence about 26°. 



The surface-markings of tins species vary considerably on different individuals. 

 The small vertical folds are usually quite obscure or wanting on the lower volutions, 

 but sometimes they are well defined even on the body-whorl; while in other instances 

 they become nearly or quite obsolete on all parts of the shell. The fine thread-like re- 

 volving lines are generally equidistant, and number about seven to ten on each whorl 

 of the spire. When well defined, they sometimes impart a slightly nodose character to 

 the folds, particularly near the middle of each whorl. Very often these revolving lines, 



