A REVISION OF THE GENUS STEGANOPORELLA. 227 



authors, with which the base of the enormous operculum is 

 continuous. In zooecia from which the epitheca has been 

 removed, the four vertical walls end in a thm '' raised line," 

 which outlines the entire zooecium, separating it from its 

 neighbours. Busk^ regarded this as a " chitinous hollow 

 filament," which he supposed to be a channel of communica- 

 tion between different parts of the zoarium. In incinerated 

 specimens the lateral walls of neighbouring zooecia may 

 appear separated from one another by a narrow slit in 

 place of the " raised line." This is in fact the edge of a 

 chitinous layer separating contiguous zocecia, and prolonged 

 into the membranous epitheca. This agrees with the account 

 given by Nitsche^ of the calcification of the zooecia of Mem- 

 branipora membranacea, in which the calcareous matter 

 is said to be formed in the middle of the chitinous ectocyst, 

 part of which is left on each side of it. In S. auriculata 

 the basal wall is covered externally by a chitinous ectocyst, 

 the limits of the zocecia being visible as a chitinous line, just 

 as on the upper surface. 



The proximal pai-t of the zooecium is covered by the 

 epitheca, which is tightly stretched across it ; and there is 

 here no calcareous portion which projects above the level of 

 the epitheca. The distal wall is usually raised above the 

 epitheca of the adjacent zooecia into a conspicuous calcareous 

 "oral arch," the development of which differs in different 

 species. 



The thin summit of the post-oral part of each lateral 

 wall will be described as the " edge." Nearer the base the 

 proximal and lateral walls have a thickened granular or 

 tubercular calcareous portion, as in many species of Mem- 

 branipora. This may be referred to as the "shelf," a name 

 which has already been used by Waters.^ It is constantly 

 present on the lateral walls, but may be evanescent on the 



1 'Challenger Report,' pt. xxx, 1884, p. 75. 

 * ' Zeitsclir. wiss. Zool.,' xxi, 1871, p. 455. 



' 'Quart. Journ. Geol. Soc.,' xliii, 1887, p. 51; Waters, however, used 

 the term only for what I describe below as the oral shelf. 



