ORDER V CHEILOSTOMATA 347 



leaves are comjjosed of two layers of cells growing in opposite directions from a mesial 

 lamina. Upper surface with solid maculae or "dimples." Lower Carboniferous. 



Family 10. Rhinoporidae Ulrich. 



Zooscia simple, ohlong or rhomhoidal, j}rone along the haml membrane; vestibules 

 direct, hemisepta iDanting ; front of zooecia beloiv vestibule usually strengthened with solid 

 or vesicular tissue. Silurian. 



Rhinopora Hall. Zoarium of undulating, bifoliate expansions ; surface smooth 

 and traversed by slender bifurcating ridges. Silurian. 



Lichenalia Hall. Like Rhinopora but unilaminar. Silurian. 

 Diamesopora Hall ; Stictotrypa Ulrich. Silurian. 



Family 11. Cycloporidae (provisional). 



The following genera, all from the Lower Carboniferous, and of doubtful affinities, 

 are placed in this family : Cyclopora Prout ; Gycloporella Ulr. ; Proutella Ulr. ; 

 Worthenopora Ulr. 



Order 5. CHEILOSTOMATA Busk. 



(Bryozoaires cellulines, d'Orlngny). 



Zooecia oval, turbinate, urceolate, qtiadrate or hexagonal, arranged usually side by 

 side. Orifice more or less anterior, of smaller diameter than the zocecium, closed by a 

 movable cover. Ova commonly matured in external marsupia. Appendicular organs 

 frequently present. 



The earliest Cheilostomata appear sparsely in the Jura of Normandy, but their 

 progenitors are undoubtedly to be looked for in the Paleozoic Cryptostomata. They 

 attain an astonishing development in the Upper Cretaceous, and in the Tertiaries and 

 existing seas they greatly surpass the Cyclostomata in number and variety of species. 



Not all of the Cheilostomata have a completely calcified zoarium, some being 

 corneous and flexible (Flustridae), and others having the front wall of the zoa3cia 

 more or less memliraneous and the rest calcareous (Memliraniporidae). Consequently, 

 in fossil examples of the latter, the zooecia are entirely open on the upper or front 

 side (Fig. 477). Avicularia and vibracula are very commonly present, and are 

 indicated in fossils by the " special pores " in which they were lodged. External 

 ovicells are more commonly developed than in the Cyclostomata, and usually occur as 

 rounded, blister-like cavities in front of the zocecial apertures. Eeproduction by 

 gemmation takes place at the growing edge of the colony, the young cells arising 

 from the anterior end or from either side of the parent cell ; and repeated gemmation 

 almost always results in a more or less regular arrangement in series. Direct 

 commvmication between adjoining zooecia is effected by means of small perforated 

 plates {communication plates, Rosettenplatten), set in corresponding positions in the 

 side walls of each zocecium. 



In the classification of the Cheilostomata, the presence or absence of the compensa- 

 tion sac is of great importance. This is a thin- walled sac opening outward through 

 a pore, the ascopore, and provided with muscles whose contraction distends the sac 

 with the result that the polypide is extended. 



Although much work has been done in recent years upon the morphological and 

 systematic study of the Cheilostomata, their classification, particularly the genera of 

 the fossil forms, is still in an unsettled condition. The older systems of d'Orbigny 

 and Busk were highly artificial, undue prominence having been given to zoarial 

 modifications ; but through the labours of Smitt, Hincks and Waters, who have 



