286 



ZOANTHARIA. 



Of the many genera of this family the three leading types are 

 Cyathophyllum, Lithostrotion, and 0?nphyma. 



In the genus Cyathophyllum itself, the corallum may be simple or 

 composite, the compound forms being sometimes massive (fig. 162), 

 sometimes fasciculate. The septa are numerous, alternately long and 



short, the longer ones being 

 continued to, or nearly to, 

 the centre of the visceral 

 chamber, where they are 

 usually more or less twisted, 

 and give rise to a sort of 

 spurious columella (fig. 

 161, a). The external dis- 

 sepimental zone is largely 

 developed, and is com- 

 posed of oblique vesicles 

 of small size (fig. 161, B) ; 

 while the internal tabulate 

 area is comparatively small 

 and incompletely devel- 

 oped, the tabulae having a 

 vesicular character. The 

 " fossula" is small or obso- 

 lete, the symmetry in the 

 latter case becoming more 

 or less completely radial ; 

 whereas when the fossula is recognisable, the symmetry is obviously 

 bilateral. Many species of the genus are known, the earliest forms 

 appearing in the Ordovician and the latest in the Carboniferous rocks. 

 The genus is especially well represented in the Devonian and Carboni- 

 ferous rocks. 



The Silurian and Devonian genus Actinocystis differs from Cyatho- 

 phyllum proper, chiefly in the fact that the septa are imperfect, passing 



Fig. 162. — A specimen of Cyathophyllum hexagonum, 

 from the Devonian Limestone of Gerolstein, of the natural 

 size. (After Zittel.) 



Fig 163. — Arachnophyllum (Strombodes) 

 pentag07iimt. Silurian, Canada. 



Fig. 164. — Ararhnofihyl- 

 lum (Strombodes) gracile. 

 Silurian, Canada. 



externally into vesicular tissue, which also largely replaces the central 

 tabulate area. Allied genera are the Spongophyllum and E?idophyllum 

 of the Devonian rocks, in which the corallum is composite, and the septa 

 are also more or less incomplete. 



The genus Acervularia, as based on the A. ananas of the Silurian 



