128 



DR. J. E. GRAY ON PLACOSPONGIA. 



[Jan. 24, 



axis covered externally with separate lamiiise ; the axis and laminae 

 composed of closely adherent siliceous globules with a granular sur- 

 face, and separated from each other by a layer of sarcode armed 

 with siliceous spicules. 



The genus may be thus described : — 



Placospongia. 



The sponge hard, angular, stony, angularly branched. The axis 

 solid, formed of closely packed siliceous globules with an areolated 

 tubercular surface, and covered with variously shaped hard plates of 

 similar tubercular siliceous globules, having an areolated appearance 

 on the surface under the mici'oscope. The outer plates differ 

 greatly in size and form ; but they meet at the edges, and rarely one 

 edge slightly overlaps the other, giving the sponge an angular ap- 

 pearance. The axis is separated from the superficial plates by a 

 continuous layer of sarcode furnished with bundles of nearly parallel 

 pin-shaped spicules, which form columns diverging at right angles 

 from the outer surface of the axis to the inner surface of the outer 

 plates. The external plates are increased in size by the addition of 

 new matter on the circumference, leaving indistinct concentric lines 

 of growth on the outer surface. It is the manner of growth that 

 makes them look so like the fronds of a large Melobesia. 



Fig. 1. Placospongia melobesioides, Gray. 



2. Cross fracture, showing the axis, sarcode, and outer hiiiiin.'u. 



3. Siliceous globule. 



4. Pin-shaped spicule of sarcode. 



Placospongia melobesioides. 



Var. 1 . Sponge thick, with short angular branches, chalky white. 



Hab. Borneo. 



B.M. 



