POLYZOA OR BRYOZOA. 



423 



Lastly, in the Lyropora of the Carboniferous, the ccencecium 

 resembles that of Fenestella in general features, but the net- 

 like frond is bounded by solid lateral supports, which spring, 

 like the two branches of the letter U, from a small base of 

 attachment. 



Fig. 264. Carboniferous Polyzoa. a, Fragment of Polypora dendroides, of the natural 

 size Ireland; a', Small portion of the same, enlarged to show the cells; b, Glauconome 

 fndcherrima, a fragment, of the natural size Ireland ; V, Portion of the same, enlarged ; 

 c, The central screw-like axis of Archimedipora WortJie/ii, of the natural size Carboniferous, 

 America ; <?, Portion of the exterior of the frond of the same, enlarged ; c", Portion of 

 the interior of the frond of the same, showing the mouths of the cells, enlarged. (After 

 M'Coy and Hall.) 



Another great group of Palaeozoic Polyzoa is typified by 

 the widely-distributed genus Ptilodictya, which ranges from 

 the Lower Silurian to the Carboniferous. In this genus the 

 ccenoecium is flattened, foliaceous, or more commonly dicho- 

 tomously branched. The cellules are placed obliquely upon 

 both sides of the thin flat frond, being separated by a deli- 



