PKIMORDIAL FOSSILS. 341 



mens are almost always depressed-convex, while the narrower forms 

 are more highly convex. Bnt among them there are all grades of outline 

 between the two extremes, and the variation in the convexity of the indi- 

 viduals is as gradual as the change in form ; while the texture of the shell 

 and the surface markings are the same throughout the whole. The shells 

 are all minute, seldom attaining more than a line in length by a little less 

 in width, or length and breadth subequal ; some of the larger individuals, 

 which are of the transversely broad-oval form, will measure about a line 

 and half in width by a length somewhat less. We have not been able to 

 satisfactorily see the internal muscular impressions, and have therefore 

 copied Messrs. Meek and Hayden's figure showing these features. 



Formation and locality. — In a hard quartzitic sandstone, of the Potsdam 

 formation, of dark greenish-purple color, at the headwaters of Red Canon 

 Creek, Black Hills, Dakota. 



ARTICULATA. 



TWLOBITA. 



CALYMENIDJE. 



Genus CREPICEPHALUS Owen (? LOGANELLUS Devine.) 



OREPICEPHALUS CENTRALIS. 



Plato 2, figs. 21-24. 



Crepicephalus {Loganellus) centralis Whitf., Prelim. Rept. Pal. Black Hills, 1877, p. 10. 



Species recognized only by the glabella and fixed cheeks, with a few 

 imperfect fragments of the movable cheeks. The former parts, when 

 united, are somewhat quadrangular in outline, broadest at the base and 

 gently narrowing in front ; glabella round-conical, higher than wide, 

 the anterior end somewhat sharply rounded; surface highly convex and 

 marked by three pairs of distinct lateral furrows, which are strongly 

 directed backward in their direction from the margin, and on the larger 

 specimens the posterior pair almost unite in the middle. Occipital furrow 

 distinct, extending entirely across the base of the head ; occipital ring 



