CORALS OF THE CIXCIXXATI GROUP. 205 



are flat and discoidal." (Milne Edwards and Haime, Bi'it. Foss. Corals, 

 p. 265.) 



The Cincinnati group of Ohio, in common with the Trenton and Hud- 

 son River formations elsewhere, yields a great many examples which 

 correspond with the above description in all essential respects — some 

 altogether so, others with more or less striking variations. Inform this 

 species is protean, being more or less discoidal when young, but being, in 

 the ad\|lt condition, sub-spherical, spherical, sub-pyriform, lobate, mush- 

 room-shaped, or not uncommonly resembling a cardinal's hat in figure. 

 The surface is sometimes mammillated with obtusely rounded tubercles 

 or elevations of variable height ; but quite commonly the surface is per- 

 fectly smooth. Definite groups of large sized corallites are often pres- 

 ent, but also can often not be recognized. In all the specimens I have 

 examined the calices are polygonal or sub-polygonal, generally from 

 eight to ten in one line, and without any very minute tubuli inter- 

 spersed amongst them. 



The young forms of C. petropolitanus have in their most typical condi- 

 tion the form of circular discs, flat or concave below, and more or less 

 strongly elevated and hemispheric above, a specimen six lines in diam- 

 eter having a thickness in the center of three lines. 



Besides the typical free-living examples of this species, with a flat or 

 concave base, and a concentrically wrinkled epitheca, the Cincinnati 

 group yields a number of massive lobate examples, which may be re- 

 garded as making an approach to C. pidchellus, E. and H., which they 

 strongly resemble in the characters of the surface and the corallites. 



Lastly, we meet with a number of smaller or larger, nodulated, hemi- 

 spherical, sub-spherical, or irregular masses, which agree with the typi- 

 cal examples of this species in most respects, but of which some are cov- 

 ered with corallites over their entire surface, whilst others are attached 

 parasitically to foreign bodies, and have therefore no concave under sur- 

 face. In the meanwhile, however, it does not seem advisable to separate 

 these forms from C petropolitanus. 



I am disposed to think that Lichenalia concentrica, Hall, has been 

 founded upon the epitheca of C. petrojwlitanus, whish is often of sufficient 

 tenuity to allow the bases of the corallites to be seen through it. 



Position and locality: Cincinnati group, Cincinnati, Ohio. 



