56 GUIDE TO THE FOSSIL INVEETEBRATE ANIMALS. 



Gallery X. from the Portlandian of Portland and Tisbury, showing how 

 Table-ease the skeleton of the coral has been converted into chert in 

 varying degrees. 



Fig. 25. — Zoantharian Corals of Upper Corallian Age, from Wiltshire. 

 a, Isastrxa explanata ; b, Thecosmilia annularis. Natural size. (From 

 Prestwicli's " Geology.") (See Table-case 2.) 



Table-cases In the Cretaceous Epoch, corals were scarcer in 

 1 & 2. , England, for the conditions were less favourable to their 

 growth. The faunas from the lower rocks are in Table- case 2, 

 those from the upper rocks in Case 1. From the Lower 

 Greensand comes Holocystis chgans, once regarded as the 

 only Eugose coral of later age than Palaeozoic. In spite, 

 however, of its four-rayed symmetry, it is now regarded as a 

 normal Astraeid. 



In the Gault and Chalk, the principal corals are small, 

 simple forms, for the mud of the former, and the cold depths 



Fig. 26. - Zoantharian corals from 

 the British Chalk. a and c are 

 Madreporaria Aporosa : a, Synhelia 

 Sharpei ; c, Parasmilia centralis. 

 6 is a Perforate Madreporarian, 

 Stephanophyllia Bowerhanki. Nat. 

 size. (From Prestwich's " Geo- 

 logy-") (See Table-case 1.) 



of the latter sea were fatal to reef-builders. The commonest 

 type is conical in shape, such as Smilotrochus and Parasmilia 

 (Fig. 26 c) ; some specimens of the latter have been split and 

 show the structure of the calyx clearly. Occasionally 



