Genus of Fossil ITeu-actinclliii Sj>onf/rs. I J 



tino;e ; while that which rej)hie<.'.s tho .siHoL-oiis fibrt' is, hy 

 retlectcd liyht, ot" a milky bhio colotir, and by traiisniitted 

 light brownish, k\s.s transparent, and granuhir with dark spots. 

 And thns while the fiindaniontal s])iculc has become absorbed, 

 and its liollow cast tilled with crystalline calcite, and the same 

 material has rej)laeed the siliceous fibre and the sarcode 

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

 morphosed net consists ot" 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 afliniticsof 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 imi)<)rtant. 



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 inw^ards, and is replaced by a botryoidal surface of 

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

 with a corresponding cavity on the inside ; from the botryoidal 

 exterior a fibrous crystallization of silica radiates towards 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, witli their rays 

 overlapping, ]irccisely as they do in Farrca and AphrocalUstes 

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

 equally distant from each other have been noticed surroundino- 

 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, tliese canals underwent an extension so far into the 

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

 should have a structure singularly homojdastic with that of 



*' Intomiesh," the space included between a mesh. 

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



