POLYZOA. 



197 



dichotomously branched. The cellules are placed obliquely 

 on both sides of the ccencecium, and have prominent oval 

 mouths. 



In the Secondary period, the remains of Polyzoa are abund- 

 ant, and some of the genera are still represented in recent 



Fig. 135. Entalophora cellarioides, natural size and enlarged. Oolites. 



seas. 



Nearly twenty genera are known from the Jurassic 

 Rocks, amongst which may be mentioned Idmonea, Eschara, 

 Defranria, Diastopora, Bidiastopora, (fig. 136), and Entalophora 



Fig. 136. Bidiastopora cervicornis, natural size and enlarged. Oolites. 



(fig. 135). All of these, except Eschara, belong to the group 

 of the Cydostomata, distinguished by their tubular cells, the 

 mouth of which equals in breadth the diameter of the cell. 



It is in the Cretaceous period that the Polyzoa attain their 

 maximum, nearly two hundred species being known in the 

 Chalk. Most of the Cretaceous forms belong to the 'Eschar- 

 id<z, the genus Eschara being abundantly represented. All 

 the members of this family belong to the group of the Cheilos- 

 tomatous Polyzoa, in which the orifice of the cell is not terminal, 

 and is of less diameter than the cell itself. 



In the Tertiary Rocks Polyzoa are likewise abundant, and 

 many of the genera are represented at the present day by 





