206 PALAEONTOLOGY OF OHIO. 



Ch^tetes discoideus, James. 



Plate 21, figs. 15-15C. 



Chxtetes discoideus, James ; Catalogue of the Lower Silurian Fossils of the Cincinnati 

 Group, 1871. (Named, but not figured or described.) 



Corallum free, discoid, sharp-edged, cdncavo-convex, from five to eight 

 lines in diameter, and from one to nearly two lines in greatest thickness. 

 Under surface concave, covered with a very thin epitheca, which but for 

 one or two obscure concentric wrinkles is nearly smooth, and which is in 

 general so delicate as to reveal clearly through its substance the bases of 

 the superjacent corallites. Upper surface gently convex, not exhibiting 

 any tubercles or elevations of any kind. Corallites sub-equal, the larger 

 ones usually scattered irregularly amongst the smaller ones, and rarely 

 aggregated into distinct groups. Calices with moderately thin walls, 

 polygonal or sub-circular, from eight to ten in the space of one line. 

 The ordinary corallites are not separated by any system of minute inter- 

 mediate tubuli. 



I am not sure that C discoideus, James, is distinct from the young of 

 C. petropolitanus. It is, however, a common form, and is very constant in 

 its dimensions. Apart from its discoidal, plano-convex shape, it is dis- 

 tinguished by its great comparative thinness, and the resulting short- 

 ness of the corallites, the sharp thin edges of the disc, the absence of 

 surface tuberosites and of distinct groups of large-sized corallites, and 

 the fact that the epitheca is not regulatly striated in a concentric man- 

 ner. Still less am I certain that C. dkcoideus can be kept separate from 

 Chsetetes (Nebulipora) lens, McCoy, of which, however, I have not, un- 

 fortunately, had the opportunity of examining an authentic specimen. 

 In the meanwhile, therefore, I have allowed 0. discoideus to stand, 

 chiefly upon the ground that C. lens is stated by McCoy to exhibit con- 

 spicuous and definite groups of large sized corallites (four or five coral- 

 lites in the space of one line), whilst such groups are altogether want- 

 ing or very imperfectly developed in the present species. 



Position and locality : Cincinnati group, Cincinnati, Ohio. Occurs, also, on the same 

 horizon, at Weston, near Toronto, Canada' West. 



Chjetetes filiasa, D'Orbigny ?. 



MonticuKpora JUiasa, D'Orbigny; Prodr. de PaMont., pi. 1, p. 25. 



Chsetetes filiasa, Edwards and Haime; Pol. Foss. des Terrs. Palteoz., p. 266. 



Corallum forming irregular masses, attached by their bases to some 

 foreign object. Surface more or less convex, covered with small rounded 



