of the Cheilostomatous Polyzoa. 
13 
Microporidae, Steganoporellidae, and other families in which the 
opercular wall remains membranous, but the body-cavity has been 
subdivided by a horizontal partition or cryptocyst 1 which is 
always more or less incomplete distally. The occurrence of the 
cryptocyst has resulted in a modification of the parietal muscles, 
most of which have disappeared, leaving a group on each side, 
near the distal end of the zooecium. Each group may pass through 
a special foramen (“opesiule”) in the cryptocyst, in order to 
reach the membranous opercular wall ( Micropora , etc.); or the 
cryptocyst may be specially modified in this region in relation with 
the parietal and other muscles ( Steganoporella ). 
(iii) The Cribrilinidae, including the genera Membraniporella 
and Cribrilina, constitute a group which is transitional from the 
Membraniporidae to the next main group. In some of these 
forms (and perhaps in all which are rightly referred to the Cribri- 
linidae), a process foreshadowed in many species of Membranipora 
(well seen in M. pyrula, Hincks), results in the covering of the 
membranous opercular wall by a roof formed by the overarching 
of a series of calcareous spines developed round the proximal and 
lateral parts of the aperture. These spines ultimately meet one 
another, the intervals between them remaining as slits or series 
of pores which probably admit water 2 into the subjacent space. 
The distal pair of spines may appear to articulate with the base- 
line of the semicircular operculum, but the operculum is really 
continuous with the original membranous opercular wall, and not 
with the roof formed by the overarching spines. The parietal 
muscles are arranged as in Membranipora or Flustra. The oral 
spines of certain Cribrilinidae and of many other Cheilostomes 
are probably serially homologous with the overarching marginal 
spines. It is significant that many of the Cretaceous Cheilostomes 
belong to the Cribrilinidae 3 . 
(iv) The condition found in Lepralioid or Escharine forms, in 
which the free surface is entirely calcified, is a further develop- 
ment of the Cribrilinidan arrangement. 
The calcareous front wall corresponds with the united over- 
arching spines of Cribrilina , the membranous opercular wall of 
which is represented by the floor of a large compensation-sac, 
which lies beneath the front wall, and usually has walls of great 
tenuity. This sac opens to the exterior at the proximal border 
of the operculum, which in many cases possesses a well-marked 
basal sclerite, strengthening its edge along the line where it comes 
into contact with the calcareous front wall. 
1 Cf. Quart. J. Micr. Sci., xliii. 1900, p. 228. 
2 I have not at present complete proof that this statement is correct. 
3 Cf, Canu, “ Rev. Bryozoaires Cretaces figures par d’Orbigny,” 2 e Partie, Bull. 
Soc. Geol. France, (3) xxvrn. 1900, p. 440. 
