POLYZOA 193 



round, polygonal, or irregular, and terminal and without 

 opercula, Ooecia are absent. The Trepostomata are found 

 chiefly in the Palaeozoic formations, but a few genera 

 linger on until the Cretaceous. 



Monticulipora. Zoarium generally massive or lobate, covered 

 with little raised portions called 'monticules.' Zocecia polygonal, 

 with thin walls. Ordovician and Silurian. Ex. 31. papiilata, 

 Silurian. 



SUB-ORDER III. CRYPTOSTOMATA 



The zocecia are calcareous and tubular, often with 

 transverse partitions, and often of two sizes. Avicularia 

 and vibracula are absent. The external orifices of the 

 zooecia are round, but these are not the true apertures ; 

 the latter are situated at the bottom of a tubular vestibule, 

 the round orifice of which is seen on the surface of the 

 zoarium. Probably a chitinous operculum covered the true 

 aperture, but it is never found in the fossils. Ooecia are 

 absent. The Cryptostomata range from the Ordovician to 

 the Permian. 



Fenestella. Zoarium funnel-shaped or fan-shaped. Branches 

 straight, united by cross-bars, so as to form a network. The cross- 

 bars do not bear zocecia. On each branch there is a median ridge or 

 carina, on the sides of which the zooecia occur. Openings of zocecia 

 round. Ordovician to Permian. Ex. F. plebeia, Carboniferous. 



Rhab domeson. Zoarium of cylindrical branches with an axial 

 tube to which the proximal ends of the zocecia are attached ; the 

 surface is divided into rhombic areas, arranged regularly, in the 

 middle of which are the round orifices. Carboniferous. Ex. R. 

 rhombiferum. 



SUB-ORDER IV. CHEILOSTOMATA 



The zooecia are sometimes calcareous, sometimes horny, 

 often both ; they are more or less box-shaped, never 

 tubular ; and not divided by transverse partitions. Zooecia, 



w. p. 13 



