Genus of Fossil HexactineUid Sponges. 17 



tinge ; while that which replaces the siliceous fibre is, by 

 reflected light, of a milky blue colour, and by transmitted 

 light brownish, less transparent, and granular with dark spots. 

 And thus while the fundamental spicule has become absorbed, 

 and its hollow cast filled with crystalline calcite, and the same 

 material has replaced the siliceous fibre and the sarcode 

 between the meshes — while, in fact, the whole of the meta- 

 morphosed net consists of one material, carbonate of lime, the 

 structure is yet left as definitely recorded as in a sponge with 

 its natural composition only just dead ; and from this striking 

 fact is forced upon us the conclusion that in determining the 

 characters and affinities of fossil sponges, the mineral composi- 

 tion is an argument of but fifth-rate value, and the form and 

 structure here, as in most other anatomical questions, is the 

 one thing important. 



It frequently happens that while the sponge towards the 

 exterior is preserved in calcite, it is fossilized with silica in 

 the interior ; and between these two conditions one can often 

 trace a series of transitional changes. Thus in one specimen 

 the sharp outline of the siliceous fibre soon disappears as it 

 proceeds inwards, and is replaced by a botryoidal surface of 

 hemispherical bosses (p. 18. fig. 6, a; p. 19. fig. 7, a), each 

 with a corresponding cavity on the inside ; from the botryoidal 

 exterior a fibrous crystallization of silica radiates toAvards the 

 middle of each intermesh *, filling it up ; the interior of the 

 fibre, on the other hand, is occupied with clear transparent cal- 

 cite exhibiting cleavage-planes, and the sexradiate canal is filled 

 with silica, crypto-crystalline, and exhibiting patches of colour 

 when polarized light is passed through it. Thus the original 

 siliceous spicule is, after a cycle of changes, restored again to 

 the siliceous state. And here one may notice the very impor- 

 tant fact that these pseudomorphic spicules are not continuous 

 with each other, but remain perfectly distinct, with their rays 

 overlapping, precisely as they do in Farrea and AjjhrocalUstes 

 (fig. 5, a). In one or two instances (fig. 5, h) four spines 

 equally distant from each other have been noticed surrounding 

 the proximal end of each ray, and pointing towards the centre of 

 the spicule — thus indicating that in these cases a hollow 

 process, now converted into a spine, once proceeded from the 

 central canal and entered the thickening of fibre which fills 

 up the angles at the nodes of the network. If, as might easily 

 happen, these canals underwent an extension so far into the 

 thickening as to meet one another, and become continuous, we 

 should have a structure singularly homoplastic with that of 



* " Intermesh," the space inchided between a mesh. 

 Ann. & Mag. N. Hist. Ser. 4. Vol. xix. 2 



