19^ POLYZOA 



differing from the normal forms in size and shape, and 

 modified for protective purposes, are often present, and are 

 called avicularia and vibracula — according to whether their 

 function is to pinch or to sweep away foreign bodies which 

 would settle on the zoarium. The apertures of the zocecia 

 are contracted and not terminal, of varying outline, and 

 provided with a movable operculum, which being horny is 

 not found in fossil specimens. Globular ooecia are often 

 present ; these are not modified individuals, but outgrowths 

 in front of the distal end of each zooecium. The Cheilo- 

 stomata range from the Jurassic to the present day, but are 

 rare in deposits earlier than the Chalk. 



Membranipora. Zoarium encrusting, or erect ; the top of 

 each zooecium is covered by a chitinous membrane in which is 

 situated the aperture ; consequently in fossil specimens each 

 zooecium has a rim enclosing an unroofed sj3ace ; the rim may have 

 spines around it. Jurassic to present day. Ex. M. elliptica, Chalk. 



Cribrilina. Zoarium usually encrusting. Zocecia as in Mem- 

 branipora, but the spines of the rim meet and fuse with their 

 neighbouring and with their opposite fellows, and form an incomplete 

 roof over the zooecium. Jurassic to present day. Ex. C. jukes- 

 brownei, Chalk. 



Micropora. Zoarium encrusting. Zocecia with an encircling 

 rim as in Membranipora, but the chitinous roof is replaced by one of 

 carbonate of lime ; and this roof is perforated by two holes, one on 

 each side, near the rim and proximally to the orifice. Cretaceous to 

 present day. Ex. M. cribriformis, Barton Beds. 



Cellepora. Zocecia heaped irregularly upon an irregular 

 encrusting or erect zoarium ; the front wall entirely calcareous and 

 very convex ; the aperture terminal, more or less round, always 

 accompanied by one or more small avicularia ; in addition larger 

 avicularia are often present between the normal zocecia. Tertiary 

 to present day. Ex. C. tubigrera, Coralline Crag. 



