374 GEOLOGY OF THE BLACK HILLS. 



PSAMMOBIID^]. 



Genus PSAMMOBIA Lam. 

 PSAMMOBIA ! PBEMATUBA. 



Plate 5, fig. 31. 



Psammobia ? prematura Wliitf., Prelim. Eept. Pal. Black Hills, 1877, p. 24. 



Shell small, trans versely elongated, with parallel dorsal and basal 

 margins, 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 convexity 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 greatest 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 Psammobias that it seems unnat- 

 ural 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 the Belle Fourche River, near Bear Lodge Butte, Black Hills. 



