of the Gheilostomatous 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). 



(Hi) The Cribrilinidae, including the genera Membraniporella 

 and Gribrilina, 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 Gribrilina, 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, "Eev. Bryozoaires Cretace's figures par d'Orbigny," 2 s Partie, Bull. 

 Soc. Geol. France, (3) xxvin. 1900, p. 440. 



