3 52 NORTH AMERICAN INDEX FOSSILS. 



53. B. symmetrica (Hall). (Fig. 1661.) Siluric. 

 Inner ridge horse-shoe-shaped, faint ventrally ; marginal ridge 



faint, obsolete ventrally. 



Rochester shale of New York, etc. 



54. B. lata (Vanuxem). Siluric. 

 Similar to B. symmetrica, but horse-shoe ridge very thick. 

 Abundant in Clinton sandstones of New York and elsewhere. 



55. B. obesa Ulrich. (Fig. 1665, a-c.) Devonic. 

 Anterior end of U-shaped ridge swollen ; outer ridge low, thick, 



interrupted ventrally. 



Bryozoan bed (Onondaga) of Falls of Ohio. 



Fig. 1661. Bullia symmetrica^ nat. FiG. 1662. Ceratopsis oculifera, dorsal 

 size and enlarged. (After Hall.) view, X 12, and left valve, X ^O- (After 



Jones. ) 



56. B. ungula Jones. (Fig. 1665, rf, ^.) Devonic. 



U-shaped ridge thick, marginal ridge thin, low ; differs from 

 B. lata in form of ridge and in proportions of length and height. 



Bryozoan bed (Onondaga) of Falls of Ohio. 



XXVI. Ceratopsis Ulrich. 



Distinguished from Tetradella by the remarkable process which 

 arises from the dorsal extremity of the posterior ridge. This may 

 be straight and horn-like with one of the edges toothed (C. cham- 

 bersi Miller) or expanded somewhat mushroom-like (C. oculifera 

 Hall). Six species. Ordovicic. 

 ^y. C. chambers! (Miller). (Fig. 1660, a-c.) Ordovicic. 



Spine strong, horn-like, with toothed edge. 



Black River and Trenton of Minnesota, Eden of Ohio, etc. 

 58. C. oculifera (Hall). (Fig. 1662.) Ordovicic. 



Somewhat more elongate than preceding, anterior end more 

 oblique and acutely or rectangularly pointed. Spine blunt, mush- 

 room-like, with frilled border. (Type of genus.) 



