676 THE PALEONTOLOGY OF MINNESOTA. 



[Ceratosis chambers!. 



spine-like, or a mushroom-shaped process, beaded or fimbriated along one edge or 

 around the flattened top. Free edges of carapace as in Ctenobolbina, being thick, 

 and having "false borders." 



Type: Beyrichia chambersi S. A. Miller. 



This genus is related to Ctenobolbina on the one hand and Tetradella on the other, 

 while it is distinguished from both, as well as from all known genera, by the remark- 

 able post-dorsal process. The species of Ceratopsis are all Lower Silurian and, with 

 the exception of Beyrichia hastata Barrande, a Bohemian species evidently of this 

 genus, all American. C. chambersi is rarely met with in the middle third and rather 

 commonly in the upper third of the Trenton shales in Minnesota. Recently I have 

 also detected a few specimens in the upper part of the Trenton in Kentucky, but the 

 most typical and abundant development of the species occurs in the lower two 

 hundred feet of the Cincinnati group. Variety robusta applies to a reappearance of 

 the species in the upper beds of this group in Ohio and Minnesota. C. oculifera 

 (Beyrichia, Hall,) though* very abundant, seems to be restricted to the upper one 

 hundred feet of strata exposed in the Cincinnati hills. In this form the elevated 

 process took the shape of a thick-stemmed mushroom, the gently convex cap of 

 which is beautifully fringed at the edge. A new species, which I propose to call C. 

 intermedia, occurs at the base of the Cincinnati formation near Covington, Kentucky. 

 In this the process forms a curved spine on which the fimbria is arranged in a semi- 

 circular manner, the effect being very nearly intermediate between that exhibited 

 in C. chambersi and C. oculifera. For further remarks on this genus see under 

 Tetradella. 



CERATOPSIS CHAMBEKSI Miller. 



PLATE XL VI, FIGS. 10-22. 



Beyrichia chambersi Mn.LElt, 1874. Cin. Quar. Jour. Sci., vol. i, p. 234. 

 Tetradella chambersi ULUICH. 1890. Jour. Cin. Soc. Nat. Hist., vol. xiii, p. 112. 



SIZE. Length 1.5 mm.; bigbt 1.03 mm.; thickness 0.6 mm. 

 Length 1.8 mm.; bight 1.10 mm. 



The principal distinguishing feature of this abundant species is the spine-like 

 form of the post-dorsal process. In the typical variety, of which fig. 19 is a fair 

 example, the post-medium ridge is short and small. It is so in all the Trenton 

 specimens and in the Lower Cincinnati group types of the species. Figure 22 is 

 peculiar in having the upper end of this ridge separately developed as a small 

 rounded node. It is the only case of the kind seen, and may be abnormal. 



