OF THE SPONGIAD^. 9 



of the material itself, and its combination with a thin 

 external case of silex, it presents perhaps one of the most 

 admirable natural combinations of strength, elasticity, and 

 durability. 



. The structure which I have described as prevailing in 

 Tethea cranium is not peculiar to that genus. I obtained 

 similar results from the incineration of the spicula of Geodia 

 M'Jndrewii, Bowerbank, MS., a new and remarkably in- 

 teresting species. In this sponge there appeared to be a 

 greater amount of silex secreted in the large skeleton 

 spicula than in Tethea ; while some of them after incinera- 

 tion were resolved into thin shells of silex, others withstood 

 the operation and retained their form ; and some were so 

 completely siliceous that, on plunging them into the drop 

 of water for examination while red-hot from the flame of the 

 lamp, the result was the same as if they had been solid 

 glass rods, and these were cracked and shattered in every 

 direction (Fig. 254, Plate XI). 



I submitted to the same mode of incineration a few of 

 the long siliceous spicula or fibres of Eiiplectella aspergillum, 

 Owen, burning about half of each fibre, and the result, 

 although somewhat different, was equally satisfactory. The 

 unbujned portion appeared perfectly solid, but exhibited 

 the usual trace of concentric structure. The end thoroughly 

 burned became reduced to a thin filament of densely black 

 matter like charcoal, but the junction of the burned and 

 unbumed portions were extremely interesting. At this 

 point the action of the heat upon the concentric layers had 

 separated them from each other in the form of a series of 

 thin curved flakes or coats, illustrating the concentric 

 structure in a very satisfactory manner ; demonstrating that 

 the outer coat of siliceous matter was not the only one, and 

 that probably there were several coats, each containing a 

 sufiicient amount of silex in its composition to resist 

 disintegration by incineration (Fig. 253, Plate Xl). 



On operating in like manner on the spicula of Chalina 

 oculata, Bowerbank, little or no alteration was perceptible 

 in the spicula, the inner cavity remaining the same as in 

 the unburned ones, and distinguished only by a slight 



