420 



MOLLUSCA. 



possibly really a Polyzoon, and has, indeed, been compared 

 by Sir Wyville Thomson with the living Polyzoan genus 

 Naresia. Leaving these out of account, the Polyzoa seem to 

 commence in the Lower Silurian, and are well represented 

 in the Upper Silurian, Devonian, Carboniferous, and Permian 

 rocks, but especially in the Carboniferous. None of the 

 Palseozoic 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 abun- 

 dant ; the Coralline Crag (Pliocene) deriving its name from 

 the great profusion of its Polyzoan remains. 



The Palseozoic Polyzoa are both very numerous and very 

 peculiar in their characters ; but they may be briefly con- 



Fig. 262.Fenestella Lyelli. a, Natural size ; &, Portion enlarged ; c, Cells and spines in 

 profile. From the Carboniferous rocks of Canada. (After Dawson.) 



sidered here under the head of a limited number of leading 

 types. Foremost among these is the genus Fenestella, the 

 type-form of the great group of the Fenestellidce or " Lace- 

 corals," ranging in geological time from the Lower Silurian 

 to the Permian, where it seems, so far as known, to become 

 extinct. In this genus, the ccencecium (fig. 262) forms a 

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

 attached to some foreign object. The coencecium is com- 

 posed of a number of nearly parallel stems, united to one 

 another by numerous cross-bars or " dissepiments," enclosing 



