90 THE CRAG POLYZOA. 



OF THE CYCLOSTOMATA. 



The following " synoptical arrangement " will convey a sufficient idea of the mode 

 in which, as it appears to me, the various genera of fossil and recent Cyclostomatous 

 Polyzoa may be arranged ; and it oflfers, perhaps, a classification as nearly natural as 

 seems possible with our present very incomplete knowledge of the sub-order. Great 

 difficulties attend the diagnosis of species in this class of Polyzoa, even in the recent 

 condition ; and these difficulties, far more than in the Cheilostomata, are necessarily 

 enhanced in the case of fossil specimens. 



Owing to the great comparative simplicity and uniformity of conformation in the 

 individual cells, and the absence, for the most part, of adventitious organs, such as 

 ovicells and vibracular or avicularian organs, our principal reliance in the distinction of 

 genera and species must be placed on the general form of the polyzoary, and the mutual 

 relation of the cells ; but as in many cases these vary very greatly in different portions of 

 one and the same polyzoary, it often happens, more especially in fossil forms, that it 

 is almost impossible to determine whether two apparently distinct things may not be 

 referable to one and the same species. These observations apply more forcibly, perhaps, 

 to PustuUpora, Idmonea, and Honiera, than to any other genera, but should be taken 

 into account in several others also. 



