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Show 1867.] DR. J. E. GRAY ON PLACOSPONGIA. 127 HEMICORALLIUM JOHNSONI. Corallium johnsoni, Gray, P. Z. S. 1860, p. 394. B.M. " Zoophyte parasitic on a coral."-J. Y. Johnson, MS. Hab. Madeira (J. Y. Johnson; Free Museum, Liverpool). 7. O n Placospongia, a N e w Generic F o r m of Spongiadce in the British Museum. By Dr. J O H N E D W A R D G R A Y , F.R.S., V.P.Z.S., F.L.S., &c. The British Museum received in 1851, from Admiral Sir Edward Belcher, a specimen of a hard calcareous body said to have come from Borneo ; and in the sale at Stevens's sale-room in 1852 we purchased two other specimens, from what was understood at the time to be the remaining part of the collection that had been formed by Admiral Sir Edward Belcher during the surveying voyage. The bodies have much the appearance of the underground rhizome of a plant with a number of scars whence leaves or flowering branches have separated ; but when more closely examined, it will be found that what appears to be a scar is a separate plate. And when so examined they have so much the appearance of a very large kind of Nullipore or Melobesia that, when I first observed them, I believed that they were probably corals covered with large plates of a Melobesia, differing in size and form on the various parts of the specimens, and giving them an angular appearance, caused by the overlapping of the different fronds of this calcareous Alga; and I therefore proposed to transfer them to the Botanical Collection in the British Museum. An examination by the microscope at once dispelled this idea ; for the surfaces of the white chalk-like plates, even under a low power, are seen to be distinctly areolated as if formed of small grains; and when the plates and the white chalk-like axis were more minutely examined under a higher power they were found to be entirely formed of transparent, more or less globular or oblong siliceous masses, with a regularly granulated surface, evidently formed of spicules radiating from the centre to the circumference, and forming the granular surface exactly like what are called the ovaria of Geodia and its allies. Also the space between the central axis and the plates in a transverse fracture was filled with a rugose yellow granular matter, which proved to be sarcode strengthened with bundles of siliceous pin-shaped spicules (with a distinct head and a tapering point), which diverge from the axis to the inner surface of the external plates. After this examination there could be no doubt that this was a sponge differing in internal structure and external form from any sponge yet described. I therefore propose to form it into a genus, to be called Placospongia, which I regard as the type of a new family, and, indeed, of a separate group of sponges, which may be called Stony Sponges, thus characterized:-Sponge consisting of a hard central |