72 CORALS 



elsewhere in tropical seas. In habit they resemble some 

 of the more typical Astraeid corals, being massive, dome- 

 shaped, lobate, or encrusting, and the surface is honeycombed 

 with small close-set calices without any intervening coeno- 

 steum. There can be no doubt that the old generic name, 

 Astraea or Star-coral, was hrst given to a member of this 

 genus, and it seems an unhappy fate for it to be removed to 

 another family than that to which it gave the family name. 

 A detailed examination of the structure of the coral, 

 however, proves quite conclusively that it is more closeh' 

 related to Fungiidae than to the Astraeidae, but it differs 

 from the Fungiidae sufficiently to justify the course, which 

 many authors prefer, of placing it, together with a large 

 number of extinct genera, in a separate family or sub- 

 family called the Plesio-fungiidae. 



The calices are usually quite small {i.e. 4-6 mm. in 

 diameter), and each calyx is separated from its neighbours 

 by a common thecal wall which is rounded above and ridged 

 by the outer edges of the septa. The septa are numerous 

 (36-48) and arranged in several series of magnitude, as in 

 Fungia, but it is a characteristic feature that thev are, 

 relatively to the size of the calyx, very thick, so that the 

 interseptal spaces are very narrow. The free edges and 

 the sides of the septa are beset with many coarse granular 

 tubercles, and in the lower parts of the septa some of the 

 tubercles of adjacent septa meet to form true synapticula 

 (Fig. 27). 



It is perhaps of some importance to note that in Sider- 

 astraea neither the septa nor the thecal walls are ever 

 perforated by holes, so that it is strictly an imperforate 

 coral. The calyx is considerably depressed in the middle, 

 and from the bottom of the central pit there rises a short 

 papillose or smooth but perfectly distinct columella. 



The method of asexual reproduction is very difficult to 

 understand b}' the study of the dried corallum. Small 

 young calices can be seen interposed in the angles between 

 the older ones and appear to arise from the common thecal 

 wall. In some cases it might be supposed that the young 

 calyx has arisen from an older one by a process of fission, 



