COEALS OF THE CINCINNATI GROUP. 191 



divided into the four following groups, a single species being apparently 

 sometimes, though certainly very rarely, represented by examples belong- 

 ing to two groups : 



A. Ramose Species. — Corallum ramose, or dendroid, the corallites 

 springing in a radiating manner from an imaginary axis. The base 

 rooted, the extremities of the branches rounded. Some of the forms of 

 this group are slender ; others are more or less swollen and tumid ; and 

 these latter appear to form a transition between the former and certain 

 lobate examples, which may be variously regarded as the last term of 

 the ramose group or the first term of the massive group. The species of 

 Chastetes belonging to this section which I have recognized in the Silu- 

 rian rocks of Ohio are C. Dalei, Edw. and H., 0. rugosus, Edw. and H., 

 C. approximatus, Nich., 0. attritus, Nich., 0. pulchellus, Edw. and H., O. 

 Fletcheri, Edw. and H., C. gracilis, James, C. delicatulus, Nich., C. nodu- 

 losus, Nich., C. Jamesi, Nich., C. rhombica, Nich., C. siib-pulchellus, Nich., 

 C. briareus, Nich., and Chxtetes sigillarioides, Nich. 



B. Massive Species. — Corallum fixed or free ; the form very variable, 

 most commonly concave convex or discoidal, hemispherical or nearly 

 spherical, sometimes lobate or altogether irregular in shape. The typi- 

 cal forms of this group have the under surface of the corallum covered 

 by an epitheca, and as this surface is more or less deeply concave, such 

 forms must have lived a free existence. Others are fixed to some foreign 

 object, and the mode of existence of some examples is uncertain. I have 

 recognized in the Silurian rocks of Ohio, as belonging to this section of 

 Chcetetes, the following species : C. petropolitanus, Pander, 0. discoideus, 

 James, and C.filiasa, D'Orb. (?). 



C. Feondescent Species. — Corallum forming a flattened or undulated 

 expansion, often sub-palmate or palmate, rooted at the base, but covered 

 over the entire surface on both sides by the openings of the corallites. 

 The corallum primitively consists of two layers of corallites, the bases 

 of which are opposed to each other, and which are directed in opposite 

 directions from a common calcareous membrane. In old examples, how- 

 ever, additional strata of corallites may be superimposed upon the two 

 primordial layers. In very rare instances the corallum becomes massive, 

 apparently by. the folding of the expansion and its reduplication upon 

 itself, or by the production of successive layers of corallites. The Silu- 

 rian strata of Ohio have yielded of this section 0. mammulatus, Edw. 

 and H., C.frondosus, D'Orb., and G. (?) clathratulus, James. 



D. Encrusting Species. — Corallum forming a thin crust, which con- 

 sists primitively of a single layer of corallites, and which is parasitically 

 attached to the shells of Brachiopods, Cephalopods, or other molluscs, or 



