FOSSIL ACTINOZOA. 99 



quite recently it was believed that all the Rugosa were Palaeo- 

 zoic, with the exception of the genus Holocystis, represented in 

 the Cretaceous period (Upper Greensand) by the single species 

 H. elegans. Recent researches, however, have brought to 

 light the existence in our present seas of at least two genera 

 (Haplophyllia and Guynia) which belong to the Rugose family 

 of the Cyathaxonidce ; and certain Tertiary Rugose Corals 

 have also been described (Martin Duncan). As the Rugosa 

 are in no fundamental structural character to be distinguished 

 from the Zoantharia sclerodermata, save in the number of their 

 septa, there would thus seem to be no good ground for main- 

 taining that there is any essential difference between the Palaeo- 

 zoic corals and those of more modern times. 



Recently it has been shown that some very abnormal Ru- 

 gose corals were provided with a lid or operculum, closing the 

 mouth of the calice. In the genus Calceola (fig. 56), formerly 

 referred to the Brachiopoda, and very abundant in certain parts 

 of the Devonian system, the operculum consisted of a single 

 valve or piece. In Goniophyllum four valves were present, and 

 in Cystiphyllum prismaticum there were four or more valves in 

 the operculum. It is worthy of notice that some recent corals 

 (species of Primnoa, Paramuricea, and others) exhibit also a 

 more or less complete operculum. According to Professor 

 Agassiz, the Rugosa and the Tabtilate 

 division of the Zoantharia ought not 

 to be considered as belonging to the 

 Actinozoa, but should be placed amongst 

 the Hydrozoa. This radical change, how- 

 ever, cannot be accepted without the 

 production of very conclusive evidence in 

 its favour. A strong argument against re- 

 ferring the Rugose and Tabulate Corals, Fig> 56 _^ alceola sanda . 

 as proposed by Agassiz, to the Hydrozoa, # An opercuiate Rugose 

 is their possession in most cases of well- 

 developed septa, implying, of course, the existence in the living 

 animal of mesenteries, structures which are wholly wanting in 

 the Hydrozoa. 



As regards the distribution in time of the families of the 

 Rugosa, the most important group is that of the Cyathophyllidce, 

 which is abundantly represented in the Silurian, Devonian, and 

 Carboniferous Rocks. The family Cyathaxonidce is Silurian 

 and Carboniferous, and is represented by two living genera. 

 The family Cystiphyllidce is Silurian and Devonian. Lastly, 

 the family Stauridce is represented in the Silurian Rocks by 

 the genus Stauria, in the Devonian Rocks by the genus 



