298 GILBEliT C. BOURNE. 



growth, for in its youngest stage the corallum of the nurse- 

 stock of Fungia is cup-shaped and resembles a Caryophyllia, 

 having a distinct lateral theca, and a basal disc by which it is 

 attached. In the course of subsequent growth the peristome 

 expands laterally, so that the nurse-stock already shows traces 

 of the discoid shape before any young forms are set free ; this 

 is very well seen in the specimen dredged by the "Alert/' re- 

 ferred to above. The separation of the young Fungia from 

 the nurse-stock takes place at a short distance below the edge 

 of the peristome, so that only a small portion of the lateral 

 theca of the nurse-stock passes into the free form. As growth 

 proceeds the peripheral ends of the septa are the seats of the 

 greatest activity in the deposition of calcareous matter, each 

 septum at once growing outwards and sending off calcareous 

 processes from its lower edge, which meet and fuse with those 

 of adjoining septa to form the perforate theca. Thus, the in- 

 crease in size of the coral proceeds almost entirely in a hori- 

 zontal direction, bringing about the discoid shape of the adult. 

 The corallum of a young Fungia just set free from the nurse- 

 stalk has a circular opening beneath, which leads into the 

 interseptal loculi and marks the point of former attachment; 

 this hole is soon filled up by the deposition of calcareous 

 tissue, which remains as a small boss in the centre of the base 

 of the disc, marking the space which represents the basal disc 

 of the attached coral. The remainder of the under surface is a 

 thecal structure, the more central imperforate part of which is 

 probably that portion of the lateral theca of the nurse-stalk 

 above the line of detachment, the outer and perforate part 

 being derived from a fusion of the lower ends of the septa, and 

 not intimately connected with the synapticula, as I think after 

 a careful examination of the fresh corallum, though on this point 

 I am at variance with Professor Duncan, The series of figures 

 3 — 8 show how the discoid shape is derived from the cup- 

 shaped coral by lateral growth. The theca of Fungia, although 

 entirely confined to the basal surface, and perforated by nume- 

 rous apertures leading into the interseptal loculi, is quite homo- 

 logous with the theca of other Madreporaria, and bears similar 



