24 



abundant on weathered specimens where they have apparently been 

 subject to the percolating action of mineral waters, which has partially 

 destroyed them. 



Formation and locality. — In Jurassic rocks at the top of the highest 

 hills, two miles south of Belle Pourche, and at a horizon of 350 feet 

 above the Triassic red beds, east of Belle Fourche ; and also on the east ' 

 side of Spear-fish Greek, near junction with the Red Water, northeast 

 of Crow Peak, Black Hills, Dakota. 



Fam. PSAMMOBIID.E. 

 Genus PSAMMOBIA, Lam.' 



PSAMMOBIA ? PEEMATUEA, tl. Sp. 

 Plate 5, fig. 31. 



Shell small, transversely elongated, with parallel dorsal and basal mar- 

 gins, the length equaling a little more than twice the height. Valves 

 very depressed-convex and distinctly gaping at each end. Beaks small 

 and inconspicuous, compressed, situated a trifle nearest the anterior end ; 

 the antero-cardinal line gradually declining with a very slight con- 

 vexity to the narrowly-rounded anterior end, basal margin straight for 

 more than half the length of the shell, rounding upward anteriorly and 

 posteriorly ; postero-cardinal line a very little lower than the apex of 

 the beak, behind which it is straight to near the extremity of the shell ; 

 posterior extremity reaching its grea test length considerably above the 

 middle of the valve, the margin being directed obliquely forward below 

 to the end of the basal line. Surface of the valve regularly curved 

 from beak to base and from the posterior umbonal ridge forward to the 

 anterior extremity of the shell, forming nearly a level line ; posterior 

 to the umbonal ridge the shell is depressed, forming a broad oblique 

 sulcus extending from behind the beaks to the posterior margin of the 

 shell. Muscular markings faint. Pallial line not profoundly sinuate, 

 but distinctly emarginate behind. Hinge and teeth unknown. 



The species is only recognized by single valves, but the form is so 

 precisely like that of some of the recent Fsammdbias that it seems 

 unnatural to refer it to any other genus. The parallel dorsal and basal 

 margins and the form of the posterior slope will readily distinguish it 

 from any other shell in the rocks from this region. 



Formation and locality. — In the sandy limestones of the Jurassic beds. 

 east of Belle Fourche, Black Hills, Dakota. 



GrENUS NE^RA, Gray. 

 Ne^ra longieostra, n. sp. 



Plate 5, fig. 35. 



Shell very small, scarcely exceeding one-fourth of an inch in length ; 

 transversely elongate-ovate or clavate, widest anterior to the middle, 



