12 



THE CRAG POLYZOA. 



Fig. 7. 



Both these organs, where they exist, are of very great use in the distinction of genera 

 and species ; the distinctions being drawn from the differences in position, form, and size, 

 &c., of the organs in question. They are as useful, or nearly so, in this respect in the case 

 of fossil remains as they are in that of recent species, for, although the moveable parts are 

 of course deficient in the former, the remains of the cup are always perceptible in the 

 form of openings and processes of various kinds and in various positions. 



The true nature of these openings, &c., not having been understood, they have 

 hitherto been erroneously interpreted or left wholly unnoticed, but in the following pages 

 will be found to have been copiously employed. 



In the Cyclostomata the conformation of the cell is simpler. Tt consists, as above 

 remarked, of a rigid, cylindrical, calcareous tube, whose orifice is terminal and in most cases 

 of about the same diameter as the tube. The peristome is usually simple ; sometimes, how- 

 ever, slightly expanded, thickened, emarginate, or even 

 denticulate. The orifice is never furnished with any special 

 apparatus for its closure. The surface of the cells is 

 either smooth and entire, or finely dotted and porous (fig. 7). 

 In both these sub-orders, as, in fact, in all the rest ex- 

 cept the Ctenostomata, the cells arise immediately from one 

 another, either in close contiguity or with the intervention 

 of tubular processes ; and the infinite diversity of form exhi- 

 bited in the polyzoaries is mainly due to the direction in 

 which the gemmation takes place, and which difiers in 

 almost every species. For instance, if each cell pullulate 

 at a single point at the upper and back part, a polyzoary 

 consisting of a linear series of cells, such as that of jEtea, 

 or of Hippothoa, or Crisidia will be presented ; whilst if from each cell two are given off 

 and remain in close apposition, a circularly expanded disc of greater or less regularity will be 

 produced, as in Lepralia, Fatinella, &c., and so on,^ as was long ago pointed out by M. 

 M. Edwards. 



Having thus endeavoured to indicate the general relations, and so much of the nature 

 of the Polyzoa as a class, and more particularly of the Cheilostomatous and Cyclostomatous 

 Sub-Orders, as will render the descriptive terms employed intelUgible, I will proceed to 

 the proper subject of the memoir, viz., an account of the fossil species found in the Crag, 

 commencing with the Cheilostomata, of which the families or genera may be conveniently 

 arranged as in the following synopsis.- 



' Eng. Cyclop.,' art. Polyzoa, p. 5. 



