2J2 



ZOANTHARIA. 



a fasciculate corallum, composed of unequally-sized cylindrical corallites, 

 which are not in contact laterally, or only touch each other to a limited 

 extent. According to Professor Martin Duncan, the septa are variable 

 in number and size, and the endothecal tissues consist of an abundance 

 of vesicular dissepiments along with well-developed tabulae. 



In a fourth group of Astrasans (Astrcecp ccespitosce) the corallum is 

 more or less tufted or casspitose, the corallites being produced by fission 

 from a common parent, but having their terminal portions free. As 

 examples of this large and well-marked group may be taken the recent 

 Mussa, or such Secondary corals as Thecosmilia (fig. 146) and Clado- 



phyllia, the former of these being 

 one of the most characteristic gen- 

 era of the Jurassic rocks. 



In a fifth group {Astrczce conflu- 

 entes) we have a number of well- 

 known genera of Astrasans, in which 

 the composite corallum increases 

 mainly by fission, this being so far 

 imperfect that the calices of the dif- 

 ferent corallites are usually more or 

 less completely confluent with one 

 another. Familiar examples of this 

 group are the genera Diploria (Cre- 

 taceous to Recent), M<za7idrina 

 (Jurassic to Recent), Euphyllia 

 (Jurassic to Recent), and Rhipi- 

 dogyra (Jurassic and Cretaceous). 

 All the remaining Astrasans are 

 " agglomerate " or " astraeiform," 

 consisting of numerous closely ap- 

 proximated corallites, which may 

 be united by their walls directly or 

 by ccenenchyma, and which give 

 rise by their union to massive 

 coralla. In certain of the forms 

 included in this group, such as 

 Favia (Jurassic to Recent), the 

 mode of increase is essentially by 

 fission. In others, however, includ- 

 ing many of the most typical forms 

 of the entire family, the mode of 

 increase is essentially by gemma- 

 tion, the new buds being usually 

 thrown out either from the wall or from the calice of the parent. A very 

 large number of generic types of this group are known, abounding in 

 the Secondary and Tertiary periods, and being extensively represented 

 at the present day. Among the more familiar genera may be mentioned 

 Heliastrcsa, Prionastr<za, Isastrtza (fig. 149), Septastrcea, Convexastrcea, 

 Plesiastrcea, Stylina, and Latimceandra. 



Finally, we may consider here certain forms of corals which depart 

 more or less widely from the ordinary type of the Astraeans, but which 

 are nevertheless to be regarded as belonging to this family. The first of 

 these is the recent genus Merulina, in which the corallum is composite 

 and usually foliaceous, with a basal plate which is perforated by foramina 

 and slits. In the possession of this perforated basal plate the genus 



Fig. 150. — A, A small mass of Hoiocystis ele- 

 gaus, of the natural size ; b, A few calices of 

 the same, enlarged. Cretaceous. (After Milne- 

 Edwards and Haime.) 



