106 POLYZOA. 



Membranipora. Zoarium encrusting, generally irre- 

 gular in form. Zocecia quincuncial, or arranged irregularly, 

 their margins raised, the anterior part depressed and more 

 or less membranous. Chalk to present day. 



Distribution of the Polyzoa. 



By far the larger number of the Polyzoa are marine, 

 they occur both in shallow and in deep water. The deep- 

 water forms belong mainly to the Cheilostomata, the 

 Ctenostomata being but poorly represented. The Cyclos- 

 tomata are comparatively rare at the present day, except 

 in the Northern seas. 



The earliest Polyzoa occur in the Ordovician rocks. 

 Practically all the Palseozoic genera are extinct, and 

 they belong almost entirely to the Cyclostomata. The 

 Cheilostomata do not become abundant until we reach 

 the Cretaceous, and in the Tertiary they are more im- 

 portant than the Cyclostomata. Very many of the Pliocene 

 forms belong to species still living. 



The chief genera found in the Palaeozoic are Fenestella, 

 Ptilodictya, Glauconome ; in the Jurassic, Stomatopora, 

 Spiropora, Berenicea, Idmonea ; in the Cretaceous, Cerio- 

 pora, Heteropora, Stomatopora, Membranipora, Eschara ; 

 in the Eocene, Hornera ; in the Pliocene, Membranipora, 

 Eschara, Hornera, Fascicularia. 



CLASS. BRACHIOPODA. 



In the Brachiopods the soft parts of the animal are 

 enclosed in a shell which is formed of two parts termed 

 valves, one is placed on the dorsal surface, the other on 

 the ventral. The interior of the shell is lined by a 



