284 ZOANTHARIA. 



narrowed by the removal to the Aporosa of various types originally 

 referred to this group. Thus, the Palaeozoic genera Stauria and 

 Cyathaxonia, previously generally placed among the Rugosa, are here 

 relegated to the Aporosa, while Professor Martin Duncan has been 

 followed in the reference to the same group of the Secondary genus 

 Holocystis, the Tertiary Conosmilia, and the recent Haplophyllia and 

 Guynia. As thus limited, the Rugosa constitute a moderately natural 

 assemblage of forms, which may be subdivided into three main 



Fig. 160. — PhillipsastrcEa Verneuilli. From the Devonian (Corniferous Limestone) 

 of N. America. (After Billings.) 



sections, which are typified respectively by the genera Cyathophyl- 

 lum, Zaphrentis, and Cystiphyllmn, and which may therefore be 

 spoken of as the Cyathophylloidea, Zaphrentoidea, and Cystiphyl- 

 loidea. 1 As regards their distribution in time, all these groups are 

 confined, so far as at present certainly known, to the Palaeozoic 

 period, and their characters and principal types will now be briefly 

 considered. 



I. Cyathophylloidea. 



The section of the Cyathophylloidea comprises those Rugosa in 

 which the peripheral region of the visceral chamber is more or less 

 extensively occupied by vesicular dissepimental tissue, the lenticular 

 cells of which are directed obliquely inwards and downwards, their 

 convex sides being turned upwards (fig. 161, b). The central area 

 of the visceral chamber is occupied by tabulae, but the tabulate area 

 is often much restricted. The length of the septa is very variable, 

 but these structures are always present and are generally alternately 

 long and short, the septa of each order being approximately equal. 

 In most forms there is a well-developed fossula, and the symmetry 

 is conspicuously bilateral. In others the fossula is rudimentary or 



1 The elaborate classification of the Rugose corals put forward by Dybowski 

 has not been adopted here, partly because many of the forms placed in this group 

 by this observer are here removed elsewhere, and partly because the details of his 

 proposed arrangement are largely unnatural. 



