190 Dr. G. J. Hinde on Fossil Calcispongie. 
the exterior surface of the fibre, chiefly consisting of the. 
minute spicules, now siliceous and cemented together by sili- 
ceous material. Now at first sight it might be said that in 
this sponge at least the original structure was siliceous; for 
some, though only a very small portion, of the fibre yet re- 
tains a siliceous composition. A comparison, however, with 
the present condition of siliceous hexactinellid sponges occur- 
ring in the same beds throws great doubt on such a conclusion. 
So far as my experience extends, none of the hexactinellid 
sponges in this Cambridge Greensand retain their original 
siliceous structure; their spicular skeletons for the: most part 
are now represented by hollow moulds: and on the hypothesis 
of the siliceous origin of Pharetrospongia, one would have 
expected to find the fibres dissolved away in asimilar manner, 
and the empty moulds remaining ; but this condition never 
occurs. It is true that occasionally the moulds of the hexac- 
tinellid spicules have been infiltrated with calcite; but this 
replaced material is of quite different character from that of the 
fibre of Pharetrospongia. ‘There is another fact which tends 
to prove that the siliceous spicular fil occasionally present in | 
the fibres of Pharetrospongia is of secondary origin, which is 
that it is invariably restricted to the exterior of the fibre, 
whilst the interior remains calcareous. If the fibre had been 
originally siliceous throughout, it would naturally have been 
expected that the interzor portions, those least exposed to the 
influences of change, would have retained the original struc- 
ture the longest, whereas the interior is invariably caleareous. 
On the hypothesis of the original siliceous structure, it seems 
to me inexplicable that the fibre should have remained sili- 
ceous on the exterior, and that the interior should have been 
replaced by calcite. If, however, we suppose the original 
constitution of the fibre to have been calcareous, and a sab- 
sequent silicification of the exterior surface to have taken place, 
the explanation is at once simple and easy to be understood. 
This explanation receives confirmation from the phenomena 
presented by examples of this same sponge (Pharetrospongia 
Struhani) which are abundantly met with in the Upper 
Chalk of Kent and Sussex. In these specimens the fibre is 
smooth, white, and purely calcareous in its composition. The 
form and outline are pertectly preserved, in striking contrast 
to the condition of the siliceous hexactinellids in the same 
strata, of which the originally siliceous spicular mesh has been 
completely dissolved away, leaving either hollow moulds or 
a secondary replacement of a loose powdery oxide of iron. 
Some specimens, however, of the calcareous Pharetrospongia 
have been enveloped by flints; and I have discovered that 
