1867.] DR. J. E. GRAY ON I'LACOSPONGIA. 12/ 



Hemicorallium JOHNSONI. 



CoralUum johnsoni. Gray, P. Z. S. 1860, p. 394. B.M. 



" Zoophyte parasitic on a coral." — J. Y. Johnson, MS. 

 Hab. Madeira (/. Y. Johnson; Free Museum, Liverjyool). 



7. On Placospongia, a New Generic Form of Spongiadce in the 

 British Museum. By Dr. John Edward Gray, 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 18.52 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 rhi- 

 zome 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 Nuliipore 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 diflFerent fronds of this calcareous Alga ; and I 

 therefore proposed to transfer them to the Botanical Collection in 

 the British Museum. 



x\n 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 minutelv 

 examined under a higher power they wTre 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 sur- 

 face exactly like what are called the ovar'ia of Geodia and its allies. 

 Also the space between the central axis and the plates in a trans- 

 verse 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 anv 

 sponge yet described. I therefore |)ropose 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 



