of the Gheilostomatous Poly 20 a. 
15 
Membranipora-like 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, Catenaria 
lafontii, etc. 
(v) In certain species provided with a “ median pore” ( Calwellia 
bicornis, “ E-uthyris ” episcopalis), development of the compensation- 
sac occurs as in iv (5), 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, 
Onchopo7'ella 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 Meiribranipora-Yike 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-like arrangement of their oper- 
cular wall and parietal muscles. Species of Catenicella , Catenaria , 
1 Cf. Waters, Challenger Rep., Part 79, p. 17. 
2 “British Palaeogene Bryozoa,” Trans. Zool. Soc., xm. 1895, p. 222. 
