xix DETERMINATION OF BRITISH GENERA 525 



An ovicell is a swelling in which the embryo develops, in certain 

 Cyclostomata (Fig. 237) and Cheilostomata (Fig. 241, A, o). 

 A stolon (Fig. 238, B, sty is a stem, not formed of fused zooecia, 

 from which new individuals originate. An internode, in a jointed 

 colony, is the part between any two joints. The fornix or 

 scutum (Fig. 254, A,/) is a modified spine which in some Cheilo- 

 stomata overhangs the aperture. A mucro (Fig. 255, A, mu) 

 is a spike or protuberance developed just below the orifice. A sinus 

 (Fig. 239, B, s) is a slight bay on the lower margin of the orifice. 

 The orifice opens at the upper end of the zooecium, on its front 

 surface. The length of the zooecium is the distance from the upper 

 to the lower ends, and the width the distance between its sides. 



One or more of the following characters : orifice provided with an 

 operculum ; avicularia or vibracula present ; a globular ovicell 

 above the orifice of certain zooecia (Cheilostomata) . 7 



Opercula, avicularia, vibracula, and ovicells completely absent, or 

 inconspicuous. Calcareous or non-calcareous. If calcareous, the 

 orifice is not at the end of a free cylindrical portion . 3 



Calcareous ; zooecia cylindrical, often united for the greater part of 

 their length, but usually ending in a free cylindrical portion, 

 which bears the terminal orifice. The zooecia may be much 

 obscured by calcifications surrounding their basal parts . 2 

 f Zooecia long, tubular, with a lateral membranous region at the upper 

 2 I end, given off quite separately from a creeping stolon Aetea 



' J Zooecia more or less united to one another, orifice without chitinous 

 I operculum (Cyclostomata 1 ) . . . .63 



( Zooecia without marginal spines ; arising from a branching axis, 

 which is not formed of zooecia . . . .74 



3. ! Colony adherent ; or erect, fleshy and slightly branched ; or erect, 



encrusted with earthy matter and repeatedly branched . 72 

 ^ Characters not as above . . . . .4 



( Zooecia minute, boat-shaped, united by a delicate tube. Aperture 



4. ^ large, with marginal spines Beania mirdbilis 



( Colony delicate, erect ; zooecia wider above than below . 5 



I No marginal spines . . . . .6 



5. < Zooecia uniserial, with marginal spines. Branches arising from the 

 ( top of a zooecium . . Brettia 

 f Zooecia uniserial ; branches arising just below the large aperture. An 



ovicell may be developed above the orifice of a modified zooecium 



6. ^ Eucratea chelata 



Zooecia somewhat pear-shaped ; orifice small, semicircular 



Huxleya fraf/ilis 



1 One or two genera of Cheilostomata may be mistaken for Cyclostomata. In 

 case of doubt, 7 et scq. must be worked through. 



