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Show 412 ON THE MADREPORARIAN GENUS PHYMASTRAEA. [June 19, Sections of the corallum must cut across corallites at different angles to their long axes; and the appearances presented here and there, although perfectly explicable in the perfect specimens, might be mistaken for fissiparous calicular division. The appearance 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 Phymastraa would be very isolated in the classification were the two original species the only ones ; but the new species, on which the costa; are tolerably well developed, allies it toHeliastraa. It does happen that very costulate Heliastraeans have a union between opposing costae 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 Heliastraa is a remarkable distinction, and decides the comparatively symmetrical shape of the Heliastraean calices. The genus Astraa appears at first sight to be allied to Phymastraa; 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 associated with Phymastraa, 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 requisite to leave the genus Phymastraa where M M . Milne-Edwards and Jules Haime placed it, between Heliastraa 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 Phymastraa, especially the species of Elysastraa described from the Infra-Lias of the Sutton Stone and Brocastle in South Wales. The resemblance is with the species described by M M . Milne- Edwards and Jules Haime ; and the figures given by me in the ' Monograph of the British Fossil Corals,' second series, part iv. no. 1, Palaeontog. Soc. 1867, plate vi. figs. 5-13, especially figure 10, are very suggestive. But the complete epitheca does not surround junction-processes in Elysastraa ; 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 union. |
