92 PERCY SLADEN TRUST EXPEDITION 



widest at calicular margins, up to 20 X 12 mm., average about 11 or 12 mm., decreasing 

 towards bases, depth up to 11 mm., usually about 8 mm., but much shallower in valleys, 

 and near edges sometimes quite flat. 



Septa varying in thickness, sloping, with toothed edges, sides smooth to rough, from 

 35 to 70 in number. Of these from 14 to 20 septa meeting columella, not more than 5 mm. 

 exsert, their edges with 8 to 18 closely-arranged teeth, usually becoming elongated towards 

 columella (up to 2 mm. high), round the latter tending to become upright. The broader 

 subsidiary septa usually curving towards and fusing with sides of principals. An 

 alternating cycle of rudimentary septa with entire margins present, these in adjacent 

 corallites usually meeting the main septa at angles over the walls. Columella formed of 

 twisted trabeculse, 2 or 2 '5 mm. in diameter. 



Multiplication by very unequal fission towards the calicular margins. In the marginal 

 region of a large corallite, one to four small corallites are frequently seen ; when definite 

 walls are formed round these, they simulate buds. 



The peculiar growth-form — incrusting corallum rising into sharp-ridged irregular 

 hillocks — fusion of corallite-walls, and the presence of small corallites near the calicular 

 margins of the normal individuals are striking features of this species, giving the speci- 

 mens an unique appearance. 



Two principal types, connected by gradation al forms, are to be distinguished, one in 

 which the intercalicinal walls and septa are very thin and columella openly spongy as in 

 most of the abdita specimens (PL 29, fig. 1 ; PL 35, fig. 2), the other with thicker walls 

 and septa and columella formed of closely-twisted trabecules as in Gardiner's examples of 

 PrionastroBa fusco-viridis (PL 29, figs. 3 and 4). 



Polyps. (1) Exocoelic tentacles absent. (2) Stomodseal ridges usually broader 

 than thick, constricted at their bases and with convex inner surfaces, in transverse section 

 more or less resembling those of F. pentagona. (3) About nine principal couples of 

 mesenteries. (4) In the stomodseal region of polyp subsidiary couples of mesenteries, 

 about thrice the number of principal couples. (.5) In the stomodseal region of polyp, 

 entocoelic pleats very narrow, thick, unconstricted at their bases (in some mesenteries 

 hardly recognisable) and restricted to outer quarter of width of principal mesenteries ; 

 mesenterial mesoglsea stouter than in F. favus ; pleatal region thicker than non-pleatal 

 region. (6) Mesenterial endoderm vacuolated and somewhat swollen in pleatal region 

 and near stomodseal attachment, the vacuoles being goblet-shaped, elsewhere quite thin. 

 (7) Convolutions of mesenteries massed together in inter-mesenteric chambei-s below 

 stomodseum to base of polyp. 



Remarks. A. Polyps. The poljrps follow the very irregular shapes of the corallites. 

 Owing to their approximation the coenosarc is absent. About twenty-three subsidiary 

 couples of mesenteries were counted ; some of the newly-formed couples are very narrow, 

 not extending below the stomodseal region of the polyps and without tentacles over their 

 entocoeles. The tentacular batteries resemble in shape and structure those of F. favus. 

 The stomodseum is oval in transverse section with the diameters measuring about 1"25 and 

 •75 mm. Nematocysts I and III are occasionally present in the ridges, but the mesoglsea 

 is not thickened in them. Filaments are present on nearly all the mesenteries, being 



