412 ON THE iM.\DlllCPf)H,VUlAN OENIIS I'll Y.MASTK.EA. [JuilC 19, 



Sections of the corallurn must cut across corallites at different 

 angles to their long axes ; and the appearances |)resented here and 

 there, although perfectly explicable in the perfect specimens, might 

 be mistaken" for fissiparous calicular division. The ai)pearance 

 of the sections reminds one of that of many fossil corals which 

 have weathered, or which have been partly preserved, or which are 

 offered to the student in sections. The truth could not be ascertained 

 from such relics. 



VII. The Affinities of the Genus with others of the Recent 

 Coral-fauna. 



The genus Phi/mastreea would be very isolated in the classifica- 

 tion were the two original species the only ones ; but the new species, 

 on which the cost.T are tolerably well developed, allies it io Ileliastrcea. 

 It does happen that very costulate Heliastrfcans have a union 

 between opjiosing costse by their spinulose growths, but it is a rare 

 and not invariable occurrence. The growth of the two genera is much 

 the same ; but the presence of exotheca extending beyond the costae 

 and between the corallites in Ileliastrcea is a remarkable distinction, 

 and decides the comparatively symmetrical shape of the Heliastrajan 

 calices. The genus Astrcsa appears at first sight to be allied to 

 PhymastreBu ; but a careful study of its structure indicates that its 

 junction-processes are synapticula. 



The bushy forms which increase by gemmation from the external 

 wall below the calice, and which have a more or less complete 

 epitheca, and belong to the genus Cladocora, cannot be associa- 

 ted with Fhymastraa, for when junction of corallites does occur 

 in them it is through the epithecal bands which exist here and there, 

 and not by means of mural structures. 



In classification it is therefore recpiisite to leave the genus Phy- 

 mastrcea where MM. Milne-Edwards and Jules Ilaime placed it, 

 between Jleliastraa and the genera with entirely soldered or united 

 walls. 



VIII. The Affinities with Extinct Genera. 



Some of the early Secondary corals have a superficial resemblance 

 to Phymastrcea, especially the species of Elysastrcea described from 

 the Infra-Lias of the Sutton Stone and Brocastle in South Wales. 

 The resemblance is with the species described by MM. Mihie- 

 Edwards and Jules Ilaime ; and the figures given by me in the 

 ' Monograph of the British Fossil Corals,' second series, part iv. 

 no. 1, Palseontog. Soc. 1867, plate vi. figs. 5-13, especially figure 

 10, are very suggestive. But the complete epitheca does not sur- 

 round junction-processes in Elysastrcea ; they do not exist. In the 

 genera more or less allied to Cladocora, and which are found fossil, 

 there are no junction-processes. The genus really stands alone in 

 its characteristic method of corallite iniion. 



