POLYZOA. 



195 



Of the above sub-orders of the Polyzoa, only the marine 

 groups of the Cheilostomata and Cydostomata are known to 

 occur in the fossil condition ; their preservation being due to 

 their marine habits and their general possession of a calcareous 

 or sub-calcareous coenoecium. The general facts as to the dis- 

 tribution of the Polyzoa in past time have been already alluded 

 to. The (OldhamiaiQf the Cambrian Rocks and the Graptolites 

 have been referred to the Polyzoa ; but the former is probably 

 a plant, and the latter almost certainly belong to the Hydrozoa. 

 Leaving these out of account, the Polyzoa seem to commence 

 in the Upper Cambrian, and are well represented in the Silu- 

 rian, Devonian, Carboniferous, and Permian Rocks, but espe- 

 cially in the Carboniferous. None of the Palaeozoic genera 

 extend into the Secondary period. In the Secondary period 

 Polyzoa are very abundant, and they attain their maximum 

 of development in the Cretaceous period, the Chalk having 

 yielded over two hundred species belonging to this class. In 

 the Tertiary period, also, Polyzoa are abundant ; the Coralline 

 Crag (Pliocene) deriving its name from the great profusion of 

 its Polyzoan remains. 



Of the Palaeozoic Polyzoa the most important forms belong 

 to the family of the Fenestellidce) or " Lace-corals." These 

 commence in the Lower Silurian, and extend to the Permian ; 

 but they are especially characteristic of the Carboniferous 

 Rocks. In Fenestella itself (fig. 133), the coenoecium forms a 



Fig. 133. Fenestella Lyelli. a Natural size ; b Portion enlarged; c Cells and spines 

 in profile. From the Carboniferous Rocks of Canada (after Dawson). 



funnel-shaped or fan-shaped expansion, the base of which is 

 attached to some foreign object. The coencecium is composed 

 of a number of nearly parallel stems, united to one another by 

 numerous cross-bars or dissepiments, enclosing small inter- 



