202 



FOSSIL ACTINOZOA. 



by numerous and varied forms in the Silurian, Devonian, 

 Carboniferous, and Permian rocks. The corallum is some- 

 times massive, sometimes branched, sometimes laminar, 

 and sometimes encrusting ; the corallites are prismatic, gen- 

 erally of small size, always with imperforate walls ; and the 

 tabulae are numerous and well developed. In some species 



Fig. 90. Clicetetes petropolitanus. A, A specimen viewed sideways, of the natural size ; B, 

 A horizontal section of the same, highly enlarged ; c, A vertical section of the same, greatly 

 enlarged, showing the tabulae. Lower Silurian. (Original.) 



of Clicetetes as in other members of this family some of 

 the corallites are of larger size than the others ; this prob- 

 ably indicating that the colony was composed, in its living 

 condition, of two distinct and different sets of zooids. 



The genera Prasopora, Dania, Dekayia, and Constellaria, are 

 nearly allied to Chcetetes, and are all Silurian. Beaumontia, 

 closely resembling Favosites in form, is Carboniferous. It is 

 probable, also, that we should include in this family the 

 closely allied or identical genera, Fistulipora and Callopora, 

 both of which are well represented in the Silurian and 

 Devonian, but which present many striking points of like- 

 ness to the Polyzoa. 



3. Thecidce. This family includes only the single genus 

 Thecia, confined to the Silurian period. The corallum is 

 compound, septa are present, and tabula? are well developed. 

 The precise affinities of this genus are still obscure, but 

 there is a considerable probability that it should really be re- 

 garded as an Alcyonarian, and placed in the neighbourhood 

 of Heliolites. 



4. Halysitidce. In this family we have the most typical 

 of the " Tabulate Corals," or, at any rate, those which appear 

 most likely to hold their ground as a separate division of the 

 Zoantharia. The corallum is always compound, rudimentary 



