444 Mr. H. J. Carter on the 



that we have here en masse what my illustrations of the fossil 

 spicules of the same geological formation on Haldon Hill, 

 near Exeter, show in individual spicules (Annals, 1871, 

 vol. vii. pis. vii. & viii.). Thus the existence of such sponges 

 in the Upper Greensand had been predetermined. 



Observations. 



The above arrangements of the Hexactinellidaj and Lithistidas, 

 together with the short commentaries which follow them 

 respectively, must be regarded only as preliminary to more 

 detailed descriptions which I hope to offer on some future 

 occasion. They are chiefly intended as an introduction to 

 what will hereafter be stated of the ApJirocattistidce, Farrece^ 

 and Aidodictyon, concerning which I had, as before stated, 

 gathered many facts for publication hitherto unnoticed, when 

 I found it necessary to make myself acquainted with all the 

 Hexactinellida3 and Lithistidas that had been made known 

 before I could satisfactorily acquit myself of the task. 



This involved much time and much research, combined 

 with opportunities which may not readily occur again. 

 Hence I thought it desirable to record at once the most 

 important part of my observations, although this is not the 

 place to give the whole, which would entail long descriptions. 



Object. 



We now come to the primary object of this paper, which 

 was to show that the tubular lines which appear in the 

 vitreous fibre of the Hexactinellidas arise from the absorption 

 of the spicules on which it was originally deposited — that if 

 this has not gone too far, the exact forms of these spicules can 

 be recognized; so that, although nothing else but the deciduous 

 fibre remains, the species of the sponge to which it belonged 

 can thus be determined if previously known in the living 

 state — and if not previously known in the living state, then 

 also the kind of spicules it must have possessed in this con- 

 dition. The deciduous specimens of Hexactinellidge which will 

 come before us for this purpose belong to Aphrocallistes Bocagei 

 and Farrea occa, of which the living state of the former is well 

 known, but the deciduous skeleton only of the latter. Mean- 

 while, for the sake of reference, it will be necessary to 

 premise a short account of each of the specimens from which my 

 observations have been derived ; then a description of each of 

 the spicules of the species of Hexactinellidge with which we are 

 now chiefly concerned that have been found in a living state, 

 viz. the Aphrocalh'sfidce 7 Aulodictyon, and the Farrece ; and, 



