432 MOLLUSC A. 



the Cretaceous to the Eecent period) has a discoidal poly- 

 zoary, with the tubes concentrically opening on the upper 

 surface. Berenicea has the ccencecium composed of a series 

 of superimposed layers. Discoporella, of the Tertiary, is like 

 Patinella, but there is an interstitial cancellated tissue between 

 the tubes ; and Defrancia (Jurassic to Eecent) is chiefly dis- 

 tinguished from both of these by the fact that the cells are 

 disposed in distinct elevated ridges radiating from the centre 

 of the disc, 



In the family of the Cerioporidce, again, are forms with a 

 solid, simple, or branched coenoacium, composed of crowded 

 contiguous cells. In Fungella (Cretaceous and Tertiary), 

 the colony is stalked, and the cells open on its upper sur- 

 face. Ceriopora itself is an ill-defined genus, the limits and 

 range of which are not known, forms of very diverse affini- 

 ties having been included by palaeontologists under this 

 name. It is possible, however, that in the reconstitution of 

 this genus, certain of the Palaeozoic Polyzoa may be found 

 capable of inclusion in it, as a more than merely provisional 

 arrangement. A better - defined genus of this family is 

 Heteropora (fig. 259), which apparently commences in the 

 Jurassic, and is represented at the present day by a single 

 species (still undescribed) from the seas round New Zealand. 

 In this genus the polyzoary is erect, and usually more or 

 less branched, the branches being cylindrical. The cell-tubes 

 radiate outwards from an imaginary vertical axis, being ver- 

 tical in the centre of the branches and becoming nearly 

 horizontal as their mouths are approached. On the surface 

 are seen two sets of differently-sized openings, of which the 

 largest are the cell-mouths, and are much fewer in number 

 than the others. The smaller and more numerous openings, 

 on the other hand, are generally termed " ostioles," on the 

 belief that the small canals into which they lead are not 

 of the nature of true " cells," but are possibly water-canals. 

 These so-called " interstitial canals " may, however, possibly 

 have lodged a peculiar set of zooids, the colony being a 

 dimorphic one. Be this as it may, the mouths of these 

 canals (" ostioles ") are often closed at the surface by a 



