30 Mr. H. J. Carter on the 



averaging l-60th inch in diameter, is composed of opaque, 

 waxy-looking, yellowish ealcite, axiated by a core not more 

 than half its size of monactinellid spicules. (How comes this 

 calcitic investment ?) In form the spicule is slightly curved, 

 fusiform, smooth, and pointed at each end, apparently about 

 15 by l-1800th inch in its greatest dimensions, and the 

 whole soluble in nitric acid. The same appears to be the case 

 with the reticulated fibre inside the flint ; but when a fragment 

 of this has been subjected to the influence of nitric acid and 

 examined under the microscope, a small portion of the spicular 

 core in which the spicules are evident is seen to remain, show- 

 ing that in the flint it is partially silicified. 



On the other hand, in the u brown flint " from the detritus 

 of the greensand &c. a mould only of the fibre generally ex- 

 ists ; but here and there fragments of thin siliceous fibre 

 bearing the same form of spicule, only a little smaller than 

 that in the black flint, partially occupies the cavity which, 



had there been any calcite present as in the chalk-flint, might 



have been entirely filled. Thus, although not wholly, Prof. 

 Zittel's statement seems to be generally demonstrated. The 

 successively inflated form of the branch in this fragment and 

 its round extremity, together with the form of the spicules, is 

 so like an existing species of Chalina y that it is almost im- 

 possible to view it otherwise than as a fossilized specimen of 

 that kind of sponge. 



It is in the wax-like, yellowish calcitic mineralization 

 similar to what has been described in the reticulated fibrous 

 structure outside the " black " flint that the fibre of Pharetro- 

 spongia Strahani presents itself in the pure chalk, as shown 

 me in several instances by Dr. Hinde in the geological collec- 

 tion of fossil sponges at the British Museum, where also there 

 is a specimen, almost identical with that which I have de- 

 scribed in the u black flint,' * that also effervesces with acid, 

 as also shown to me by Dr. Hinde — all seeming to demonstrate 

 that these specimens have been calcareous from their origin. 

 But have they been so? for this is the " vexata quEestio." 



Let us turn our attention to what has taken place in this 

 respect during the fossilization of the Hexactinellid called 

 Acanthospongia Smithii, and the Holorhaphidote sponge Pul- 

 villus Thomsoni respectively, both from the u mountain-lime- 

 stone " of the Carboniferous series in the neighbourhood of 

 Glasgow (No. 6, vol. i. p. 128, pis. ix., x.). It may fairly 

 be inferred from the intimate resemblance in form of the 

 spicules of these two species with those of the present day, 

 together with their minute structure (which has been faith- 

 fully preserved in the limestone), that they were originally 



