310 PALAEONTOLOGY 



and the theca no longer encloses the visceral chamber, 

 common in the Wenlock Limestone ; Cyathaxonia, acutely 

 conical, with strong columella, Carboniferous Limestone. 



GROUP : Zaphrentoidea. Simple corals ; septa and 

 tabulae well developed, but few dissepiments. Includes 

 Zap.hventis, Caninia, and Amplexus, already mentioned. 

 Streptelasma (Ord.-Sil.) is usually associated with them, 

 but although earlier in time it has a more elaborate 

 structure. 



GROUP: Cyathophylloidea. Simple or compound, 

 with a marginal zone of vesicular tissue ; tabulae usually 

 less extensive than in Zaphrentoidea. 



Family Cyathophyllidce. Septa with smooth surfaces, 

 the primary lamella of each septum being thickened 

 uniformly on each face with a layer of stereoplasma ; 

 primary septa not strongly marked, symmetry approach- 

 ing radial. A large family, with chief genera : Cyatho- 

 phylhim (Sil.-Carb., Fig. 94, c), simple or compound, with 

 major septa reaching almost to centre, and minor septa 

 not very much shorter; Accrvularia, compound, massive, 

 the septa swelling out at a certain distance from the 

 centre so as to unite to form a circular "inner wall"; 

 Lithostwtion, already described ; Omphyma, simple, top- 

 shaped, with root-like outgrowths, the four main septa 

 often very conspicuous. 



Family Heliophyllidce. Septa with stereoplasma 

 deposited in vertical ridges, otherwise much like Cyatho- 

 pyllidce. Genera: Heliophyllum (Sil.-Dev.), simple; 

 Phillipsastrcea (Dev.-Carb.), compound. 



Family ClisiophyllidcB. With a large " false columella " 

 (not a solid rod, but made up of twisted lamellae, having 

 a "spider's web" appearance in transverse section, and 

 making a large protuberance in the calyx). Genera : 

 Clisiophyllum (Sil.-Carb.), simple; Lonsdaleia (Carb., 

 Visean, Fig. 94, b), compound ; Dibunophyllum (Visean), 



