68 



genus Obolella, recently published, we are fully satisfied that he is right 

 in separating these shells from the genus Oholus. 



" Our species is so closely allied to Obolella cliromatica of BilUngs, the 

 type of the genus, (see " New species of LoAver 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's 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 

 strige on the inner laminre of exfoliated specimens. 



" Locality and Position. — Central portions of the Black Hills, from the 

 Primordial or Potsdam sandstone." 



Obolella pretiosa. (N. sp.) 



a I 



Fig. 61. Fig. 62. Fig. 63. 



Fig. 61. — Obolella pretiosa. — a. Ventral valve of a small specimen; b. dorsal valve 



of a specimen of the average size. 



62. — 0. desiderata. — a. Cast of the interior of what is supposed to be the 



ventral valve ; b. cast of interior of dorsal valve. 

 63. — O Ida. — a. Dorsal, and b. ventral valve. 



Description. — Transversely broad oval ; width greater than the length ; 

 front margin broadly rounded ; lateral margins more narrowly rounded 

 than the front ; greatest width about the middle, or a little below ; beak 

 of ventral ? valve minute, pointed, apparently not depressed below the 

 greatest elevation of the shell ; apical angle very obtuse, about 150 ; 

 cardinal slope straight, or sometimes gently concave on each side of the 

 beak, for a length equal to one-fourth or one-third the whole Avidth of the 

 shell, and then imperceptibly becoming curved round to the lateral margins. 

 The other valve only differs in outline by having the apex more obtuse. 

 Both valves are moderately and uniformly convex ; the greatest elevation 

 being about the middle, or a little above it. Surface with very fine 

 squamose concentric striae. In some of the specimens a few radiating 

 striae are visible. These, however, may be in some instances due to dis- 

 tortion, as the shell is very thin. 



Some of the specimens are inequilateral, or have the umbo and beak a 

 little on one side of the median line, and thus present the aspect of a small 

 lameUibranchiate shell. 



