270 



ZOANTHARIA. 



family is represented by numerous and varied types ; a still 

 further expansion takes place in the Oolites ; very numerous 

 forms are met with in the Cretaceous, and though there is some 



Fig. 146. — Thecosmilia annularis. Coral-rag, England. 



decrease in the Tertiaries, this great family still holds its ground 

 as the most important group of the "reef-building" corals. 



The family of the Astr&idce admits of subdivision into a number of 

 minor groups, and comprises such a vast number of forms that it is 

 impossible to do more here than to allude to a few of the more important 

 types. In the first place, there is a large series of 

 forms {AstrcECE simftlices) in which the corallum is 

 simple and solitary. These simple Astraeans ap- 

 pear under many generic types in the Secondary 

 period, and have survived to the present day. 

 Well-known genera are Montlivaltia (fig. 147), 

 ranging from the Trias to the Recent period ; 

 Trochosmilia, ranging from the Jurassic to the 

 Miocene ; and Parasmilia, ranging from the Cre- 

 taceous to the present day. 



In a second group of the Astraeans (Astrcecz 

 1'eptantes), comprising such genera as Rhiza?tgia 

 and Ast?-angia, the corallum is composite, and 

 consists of short corallites budded out from basal 

 stolons or expansions. Rhizangia is Cretaceous 

 and Tertiary, while Astrangia is represented in 

 the Eocene Tertiary, and survives at the present 

 day. 



In a third group of Astraeans {Astrcece gemmantes) are comprised 

 compound forms, in which the mode of increase is " by gemmation from 

 the wall below the calicular margin" (Martin Duncan). In this group 

 are included various Secondary, Tertiary, and Recent corals (Cladocora, 

 &c), and Professor Martin Duncan also places here the remarkable 



Fig. 147. — Montlival- 

 tia caryop/iyllata, show- 

 ing the greatly-developed 

 epitheca covering the 

 lower part of the coral. 

 Great Oolite. 



