176 Prof. H. A. Nicholson — On Lower Silurian Chcetetes. 



usually elevated into a central boss, but is otberwise slightly 

 convex. The largest example I have seen is about three inches 

 and a half in diameter, and has a thickness of four lines near 

 the margin, and of one inch in the centre ; but Mr. Billings mentions 

 examples which had a diameter of six inches. 



There can be no hesitation in considering the above forms as the 

 type of C. petropolitanus, aud the only question is whether we are to 

 consider a free habit aud the possession of a base covered with a 

 concentrically-wrinkled epitheca as essential characters of the 

 species or not. This question is forced upon us by the existence of 

 two other groups of forms which in many respects agree with 

 C. petropolitanus as above described, but do not possess the two 

 characters just alluded to. One of these groups, commonly repre- 

 sented in the Hudson River Group of Canada and in the Cincinnati 

 Group of Ohio, comprises forms which have often been spoken of 

 as " puff-ball varieties of Stenopora fibrosa." These forms occur 

 as small, spherical, sub-spherical, nodulated, or irregular masses, 

 which exhibit no indications of a concave base, or of an epitheca. 

 The calices seem to cover the whole surface, and the base is indi- 

 cated in fractured specimens by the radiation of the coral lites from 

 a point. Some specimens appear to have been attached to Cri- 

 noids, the column of which traverses their centre ; but others 

 exhibit no phenomena which would enable us to assert that they 

 were attached to any foreign body. In their surface-characters 

 these forms present nothing special ; but so far as I have observed, 

 they exhibit no tubercles, nor any groups of large-sized corallites. 



The second group comprises small, hemispherical, obtusely conical, 

 or sub-spherical masses, which are extremely like the typical forms 

 of C. petropolitanus, but differ in having the flattened or concave 

 base firmly attached to some foreign body, such as the shell of a 

 Brachiopod, or the column of a Crinoid. The calices in well- 

 preserved examples are thin-walled and polygonal, with minute 

 . intermediate tubuli, and distinct groups of large-sized corallites, but 

 entirely without surface tubercles. These forms occur in abundance 

 in portions of the Cincinnati Group of Ohio ; and I should be dis- 

 posed to regard them as a variety of C. petropolitanus. I do not, 

 however, feel at all so certain about the affinities of the " puff-ball " 

 forms, which are so common in parts of the Hudson Biver Group of 

 Canada. 



It may be noticed here that none of the forms here considered 

 can be referred to Chcetetes, as this genus is defined by Lonsdale and 

 McCoy. They, none of them, can be proved to increase by fission, 

 and, in all, a rough fracture exposes the walls of the corallites. 

 Neither, again, can the}' properly be referred to Stenopora, as they 

 exhibit none of the essential characters ascribed by Lonsdale to this 

 genus. Those, therefore, who refuse to extend the limits of Chcetetes 

 as defined by Lonsdale, will be compelled to place all the above- 

 described forms under Monticulipora, D'Orb. 



Chcetetes undulatus, Nicholson. 



"What forms were included by Mr. Say under the name of Favosites 



