1 84 INVERTEBRATE PALAEONTOLOGY 



is rare in low zones of the Chalk, but abundant at 

 higher horizons. Micr aster (PI. xiii. fig. 6) is especially 

 dominant in the Upper Chalk. Two zones are "named" 

 after species of the genus; but without any attempt 

 at specific determination it is possible to recognize the 

 precise horizon of any Micraster by its form and surface- 

 details. The genus has supplied some of the most 

 satisfactory material available for tracing the evolution 

 of continuous series living amid uniform surroundings. 



(F) POLYZOA 



Apart from a few representatives of the Ctenostomata, 

 Mesozoic Polyzoa were either Cyclostomata or Cheilo- 

 stomata. The former order showed a marked re- 

 awakening of specialization as compared with its Upper 

 Palaeozoic dormancy. The Cerioporina perpetuated 

 many of the characters of the Ceramoporoidea (from 

 which they may have been derived), and encrusting 

 forms, such as Berenicea and Idmonea, spread abundantly 

 over the shells and rock-surfaces of the era. It is quite 

 exceptional to collect specimens of Chalk Irregular 

 Echinoids that are free from colonies of those forms. 

 The Cheilostomata (which are probably direct de- 

 scendants of the Cryptostomata) first appeared in the 

 Jurassic, but did not become abundant until the Upper 

 Cretaceous. Sheets of Membranipora are commonly 

 found associated with encrusting Cyclostomata on Chalk 

 fossils, and free-growing types are constant, though 

 fragmentary, ingredients of " flint meal." 



Perhaps the most important Polyzoan fauna in the 

 British Mesozoic is that of the Faringdon " Sponge- 

 Gravels" of Aptian age. In most parts of this local 

 facies of " Greensand " the Sponges are subordinate to 

 Cyclostomatous Polyzoa in quantity and variety. The 



