EXPLANATION OF PLATE XV.
Discussion.
Prof. Morris expressed his agreement with the author's concluding remarks, and thought that the discovery of these low parasitic organisms so far back in geological time was especially interesting, as showing the identity of conditions in all periods. He remarked that another tubular structure different from those described by Dr. Duncan had been observed by M'Coy in Pterinea demissa, from the Wenlock Limestone, and referred to Cliona. This latter structure had also been observed in some Jurassic and Cretaceous Mollusca.
Mr. Etheridge agreed with Prof. Duncan in regarding these parasites as Algae, and remarked especially the similarity of the phenomena to those presented by many freshwater Algæ, referring especially to Vaucheria. The continuity of conditions of life thus manifested was, he thought, very remarkable. He stated that numerous sections of Scandinavian palæozoic Corals given to him by Sir Roderick Murchison were full of tubular forms like those described in the paper.
Prof. Ramsay asked Prof. Duncan to state how much of the contents of his paper was to be regarded as new.
The Author, in reply, suggested that a similar question might just as well be put to the author of every paper. His new points were the establishment of the extension of these parasitic organisms to a much earlier period than any in which they had previously been recognized, especially their occurrence in the very ancient Corals to which he had referred, and certain particulars as to their characters and mode of occurrence. The parasitic Algæ could not be con- founded with such sponges as that found by M'Coy or with Cliona, as their tubules were very much smaller than those formed by the sponge; in fact the tubules of the parasitic plants had about the same diameter as the spicules of Cliona.
