436 PROCEEDINGS OF THE ACADEMY OF 



had seen showing the interior, seemed to present differences from the type of 

 Echwald's genus. Since seeing Mr. Billings's figures of his genus Obolella, 

 recently published, we are fully satisfied that he is right in separating these 

 shells from the genus Obolus. 



Our species is so closely allied to Obolella chromalica of Billings, the type of 

 the genus, (see ''New species of Lower Silurian Fossils," page 7,) that we 

 were inclined to regard it as specifically identical, until we had an opportunity 

 to compare it with specimens of Mr. Billings' species, kindly sent us by him. 

 On comparing it with these, we find it is more convex, and proportionally 

 broader, while its concentric markings are stronger. The substance of its 

 shell is also thicker, and differs in showing radiating strife on the inner 

 laminae of exfoliated specimens. 



Locality and position. Central portions of the Black Hills, from the Primor- 

 dial or Potsdam Sandstone. 



PTEROPODA? 



Genus THECA, Morris. 



Theca (Pcgiuncdlus) gkegaeea. 



Shell small, straight, acutely conical ; dorsal side compressed or nearly flat ; 

 ventral side regularly rounded ; lateral margins obtusely subangular, and 

 converging regularly, at an angle of about 18 to the pointed lower extre- 

 mity ; aperture nearly semicircular, or forming rather more than half a cir- 

 cle ; surface smooth. 



Length, - 45 inch ; breadth, 0*15 inch ; convexity, 0*10 inch at the aperture. 



This frail little shell must have existed in vast numbers, since on a single 

 slab not more than six by eight inches across, we have counted near two hun- 

 dred individuals. It resembles in form Pugiunculus striatutus, of Barrande, 

 (Neues Jab. p. 554, pi. 9. fig. 2,) but differs in being very much smaller, and 

 much more convex and straighter on the ventral side ; as well as in being desti- 

 tute of strife. It agrees more nearly in size, and the smoothness of its surface, 

 with P. simplex, of the same author, but is smaller, and differs in having its 

 lateral margins straight instead of concave in outline. Its ventral side is 

 also much more convex, and more rounded. 



In size and form it is much more nearly like Salterella obtusa, of Billings, 

 (New sp. Lower Sil. Foss., p. 18;) but it seems to differ in not being com- 

 posed of successive layers, as in that species and genus. We only place it 

 provisionally in the genus Theca. 



Locality and position. Near the head of Powder River, in Big Horn Moun- 

 tains. From Primordial or Potsdam Sandstone. 



TRILOBITES. 



Genus APJONELLUS, Barrande. 



Akionellus (Crepicepaujs) Oweni. 



Head forming more than a semicircle, or nearly semielliptic in outline, its 

 length equalling about two-thirds its breadth, rather distinctly convex ; pos- 

 terior border more or less concave in outline, and provided with a deep rounded 

 marginal groove along each lateral slope. Glabella rather gibbous, elevated 

 above the cheeks, oblong-subovate, its lateral margins converging with a 

 slight convexity towards the front, which is rounded ; greatest convexity near 

 the middle and posteriorly, separated from the cheeks on each side and in 

 front, by a distinct sulcus ; neck furrow passing entirely across, but slightly 

 deaper on each side than at the middle ; lateral grooves three, very short, ob- 

 scure, or (in casts) nearly obsolete, not oblique. Cervical segment somewhat 



[D 



ec. 



