of the Cheilostomatous Polyzoa. 15 



Membranipora-\\ke opercular wall is postponed until it is formed 

 as the floor of the compensation-sac. The existence of this sac 

 can be easily demonstrated in the adult zooecia of various species 

 of Schizoporella and Catenicella, in Urceolipora nana, Gatenaria 

 lafontii, etc. 



(v) In certain species provided with a " median pore" (Calwellia 

 bicornis, "Euihyris" episcopalis), development of the compensation- 

 sac occurs as in iv (b), except that the sac opens to the exterior by 

 means of the median pore. It opens in the same way in the adult 

 zooecia of Calwellia (Onchopora) sinclairii 1 , Ichthyaria oculata, 

 Onchoporella bombycina, Busk, Urceolipora dentata, and probably 

 in Microporella malusii. In all these cases except in 0. bombycina 

 and U. dentata (in which suitable preserved material was not 

 available) the parietal muscles inserted into the sac have been 

 demonstrated. The view supported by Gregory 2 that the median 

 pore has been formed by the closure of the calcareous sinus which 

 receives the tongue-like projection of the operculum in Schizo- 

 porella, Urceolipora nana and others is probably correct. There is, 

 however, no essential difference between the anatomy of Lepralia 

 and that of Schizoporella ; and the validity of Gregory's divisions 

 Holothyriata and Schizothyriata, characterised respectively by the 

 absence and the presence of a sinus or median pore, is not sup- 

 ported by the study of the compensation-sac. 



(vi) The genera Scrupocellaria, Menipea, and Caberea contain 

 species which are provided with the so-called " scutum " or "fornix," 

 which is a large spine, expanding from its base, which overarches 

 the Membranipora-like opercular wall. The remarkable fact that 

 the scutum is present or absent, well-developed or small, in each 

 of these genera suggests that Scrupocellaria and its allies are 

 forms in which the membranous opercular wall became protected 

 by a single spine instead of in the manner characteristic of the 

 Cribrilinidae. The scutum is probably to be regarded as a ves- 

 tigial structure in those species in which it is small or absent. 

 The high development of avicularia or vibracula (or both) in 

 these genera confirms the view that they are not to be regarded 

 as a very primitive group of Cheilostomes. 



(vii) The foregoing considerations indicate that a complete 

 rearrangement of the Cheilostomata is required. The division 

 Cellularina in particular is an unnatural one, and consists of 

 Cheilostomes belonging to several distinct groups which have 

 taken on a dendritic habit of growth. The Bicellariidae, for 

 instance, retain a Membranipora-\ike arrangement of their oper- 

 cular wall and parietal muscles. Species of Catenicella, Gatenaria, 



1 Cf. Waters, Challenger Rep., Part 79, p. 17. 



2 "British Palaeogene Bryozoa," Tram. Zool. Soc, xin. 1895, p. 222. 



