458 Mr. H. J. Carter on the 



place so rapidly in many instances that I have for some time 

 past ceased to mount any more calcareous spicules, and now 

 preserve a record of them by immediate sketches. 



Lastly, we come to the peculiar kind of destruction to which 

 I have so often before alluded, which takes place in the centre 

 of the vitrified fibre of the HexactinellidaB apparently only 

 after the death of the sponge. This also, as before stated, 

 consists in an absorption of the spicules over which the vitreous 

 fibre was originally deposited, together with a certain amount 

 of the fibre itself, leaving nothing but their moulds, which, if 

 the absorption has not gone too far, will present exact repre- 

 sentations of the spicules respectively. It is analogous to that 

 which takes place in the spicules of the keratose sponges above 

 mentioned. 



We have here then an explanation of Dr. Bowerbank's 

 " fistulose siliceous fibre," also a proof that the siliceous fibre of 

 the Hexactinellidas is based upon the spicules of the sponge, 

 and, finally, means of detecting what the isolated spicules of the 

 sponge were, although nothing may be left but the vitreous 

 fibre in a deciduous state. It is thus that specimens of Aphro- 

 callistes Bocagei have been identified, and some of those pos- 

 sessed by Farrea occa in its living condition recognized — facts 

 which first drew my attention to the subject, originated this 

 contribution, and will now be severally described. 



The first specimen that attracted my notice in this way 

 was the bunch oiAphrocallistes in the British Museum, already 

 stated to have been dredged up by Mr. Kent on the coast of 

 Portugal ; but possessing the feature which had led Dr. 

 Bowerbank to the idea that there existed " fistulose siliceous 

 fibre " as well as " fistulose kerataceous fibre," and that this 

 was an especial characteristic of his Farrea occa, I at once 

 concluded that this was not an Aphrocallistes, but a Farrea 

 (PI. XVI. fig. 1). 



Soon, however, it became evident that this " fistulose " 

 appearance arose from the presence of sexradiate spicules 

 originally enveloped in the vitrified fibre (fig. 1, b b) ; and 

 chancing to meet with a fragment in which the characteristic 

 scopuline shaft of Ap>hrocallistes Bocagei was present (fig. 2, b) } 

 the origin of the fistulose appearance was explained, and the 

 specimen, which otherwise bore the character of Aphrocallistes, 

 shown to be not Farrea, but Aphrocallistes Bocagei with the 

 same fistulose appearance as the fibre of Farrea (fig. 4). 

 Hence the necessity, to which I have alluded, of a minute and 

 accurate description of all the spicules of these sponges. 



Subsequently the specimen dredged up on board H.M.S. 

 ' Porcupine ' at . station " No. 36 " {vide antea) came before 



