308 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.xxxv. 



Genus CERATOPSIS Ulrich. 



(''■rati Jin Ulrich, Jour. Cincinnati Soc. Nat. Hist., XIII, 1S90, p. 113 (not 



established). 

 Geratopsis T'i.rkii. Geol. and Nat. Hist. Surv. Minnesota, Final Kept., Ill, 



l't. 2, 1894, i». 675. 

 Beyrichia (part) of Authors. 



Carapace essentially as in Tetradella except that the post-dorsal 

 end of the marginal ridge is raised into a strong, spine-like, or a 

 mushroom-shaped process, which is commonly beaded or fimbriated 

 along one edge or around the flattened top. Free edges of carapace 

 blunt, the contact line between the two valve- concealed by well- 

 developed false borders. 



Genotype.— Ceratopsis chambersi (Beyrichia chambersi Miller). 



This genus stands to-day in essentially the same position given 

 it in the original definition. The species referred to it then are still 

 retained, and few new species or varieties of the genus have been 

 discovered in the past fourteen years. Among the specimens then re- 

 ferred to the genotype several minor varieties might have been dis- 

 tinguished, but the propriety of doing so was not appreciated at the 

 time. At least one. and probably two, of these varieties seem to be 

 confined to different stratigraphic horizons. Their discrimination, 

 therefore, in a subordinate degree might be desirable on this account 

 alone. However, as the prime object of this paper is to discuss 

 generic rather than specific and minor variations, it is thought ad- 

 visable to defer such matters to a time when it will be possible to treat 

 the subject monographic-ally. 



In addition to ('. chambersi, the genus includes C robusta Ulrich, 

 C. intermedia Ulrich, ('. oculifera Hall, all American upper Ordovi- 

 cian specie-, and G. hastata (Beyrichia hastata Barrande), a Bohe- 

 mian species of similar age. Beyrichia rostrata Krause, from 

 Ordovician drift in northern Germany, seems referable to Ceratopsis 

 rather than Tetradella. The same is to be said of Beyrichia quad- 

 rifida, described by done- as from the "Trenton" at Lorette Falls, 

 Canada. In the figured specimen of the latter the horn is evidently 

 broken away. In the former, providing Krause's figures represent 

 the species fully and accurately, the horn is not so well developed as 

 in the Cincinnati species. Assuming that the last two are correctly 

 understood, then the genus, as at present known, comprises seven spe- 

 cies, ranging in time from about Black River to the close of the 

 Ordovician. Apparently the stock became extinct Avith the close of 

 this period. 



