POLYZOA 195 



Distribution of the Pohjzoa 



By far the larger number of the Polyzoa are marine ; 

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

 water forms belong mainly to the Cheilostomata ; a few 

 Ctenostomata occur at considerable depths, but the group 

 is characteristic of shallow water. The Cyclostomata are 

 comparatively rare at the present day, except in the 

 Northern seas. The conditions under which the extinct 

 Trepostomata and Cryptostomata flourished best are not 

 known. 



The earliest Polyzoa occur in the Ordovician rocks. 

 Nearly all the Palaeozoic genera are extinct ; they belong 

 mainly to the Trepostomata and Cryptostomata. The 

 Cyclostomata are represented by a few genera in the 

 Palaeozoic rocks, and become increasingly abundant in the 

 Mesozoic, attaining their maximum in the Upper Cretaceous. 

 A few Cheilostomata have been recorded from the Jurassic 

 rocks, but the group does not become abundant until the 

 Cretaceous period ; in the Tertiary it is better represented 

 than the Cyclostomata. Very many of the Pliocene forms 

 belong to species which are still living. 



The chief genera found in the different systems are : — 



Palaeozoic. Archimedes, Fenestdla, Hemitrypa, Monticulipora, 

 Pinnatopora, Polypora, Ptilodictya, Rhabdomeson, Thamniscus. 



Jurassic. Berenicea, Ceriopora, Diastopora, Entalophora, Hap- 

 looscia, Idmonea, Proboscina, Spiropora, Stomatopora. 



Cretaceous. Cribrilina, Crisina, Diastopora, Entalophora, Hetero- 

 pora, Lumdites, Membranipora, Onychocella, Proboscina, Stoma- 

 topora. 



Eocene. Hornera, Idmonea, Membranipora. 



Pliocene. Alveolaria, Cellepora, Hornera, Lepralia, Membrani- 

 pora, Theonoa. 



13—2 



