CHEILOSTOMATA. 



633 



of " avicularia " ?) may be developed in the interzooecial spaces (figs. 

 479, a, and 455, c). 



In the genus Rhombopora (fig. 478) the zocecia radiate in all directions 

 from an imaginary axis, and the proper cell-apertures are placed at the 

 bottom of oval or rhomboidal "vestibules." The species of this genus 

 range from the Silurian to the Carboniferous inclusive. The genus 

 Rhabdomeson is principally, if not exclusively, Carboniferous in its range, 

 and comprises forms which are essentially similar to Rhombopora in 

 general characters, but differ in the fact that the zocecia radiate in all 

 directions from an axial calcareous tube (fig. 479, b) running up the 

 centre of the stems. 



Sub-Order II. Cheilostomata. 



In this sub-order are included all those Gymnolaematous Polyzoa 

 in which the aperture of the cell is subterminal, of less diameter 

 than the cell itself, and usually closed by a movable lip or operculum 

 (fig. 480). The essential char- 

 acter of the Cheilostomata is, 

 therefore, the position of the 

 cell-mouth on the anterior face 

 of the zocecium, instead of at 

 its extremity. The aperture 

 can also be usually closed by 

 a semicircular movable lid, 

 which may or may not be 

 calcified, and which is almost 

 always destroyed in fossilisa- 

 tion. The extent to which 

 calcification of the ectocyst 

 occurs in the Cheilostomata 

 varies greatly in different types, enlarged. 



' J r a, immer 



Sometimes the polyzoary re- 

 mains completely corneous (as in the Flustridce), in which case it is 

 incapable of preservation in the fossil condition ; whereas in other 

 cases the entire cell-wall may be calcified. There are many forms, 

 however, in which an intermediate state of parts obtains, the hinder 

 and lateral portions of the cell being calcified, while a larger or 

 smaller area of the anterior wall, particularly in the neighbourhood 

 of the mouth, remains in a membranous or horny state. Hence, in 

 such cases the cells, in the fossil condition, are more or less largely 

 open in front. In other cases, where the polyzoary is incrusting, 

 the posterior wall of the cells may be uncalcified. 



"Avicularia" and "vibracula" are' commonly developed in the 

 Cheilostomata, and in the case of the living forms afford valuable 

 characters in classification. In fossil forms, however, these minute 



Fig. 480. — Cells of Cheilostomatous Polyzoa, 

 Ectocyst ; /, Operculum ; o, Ovicell ; 

 immersed avicularium. (After Busk.) 



