188 Dr. G. J. Hinde on Fossil Calcispongic. 
calcareous spicules have been, by some authorities, compared 
to the spicules of siliceous monactinellid and _tetractinellid 
sponges whose original constitution is now replaced by cal- 
cite. Hitherto the spicules of fossil calcisponges have never 
been noticed in the free state, and our knowledge of the 
minute structure of the Pharetrones has been solely derived 
from the study of thin microscopic sections. 
Leaving aside for the present the consideration of the true 
characters and affinities of the spicules of the Pharetrones, it 
is worth while to inquire if the character of the strata in which 
these fossils occur, and the condition of other fossils associated 
with them, will give any subsidiary help towards solving the 
question of their original structure. As nearly all the Pha- 
retrones which are met with in this country are derived from 
the Lower Greensand, the Upper Greensand, and the Upper 
Chalk, we need not in making our inquiries go beyond these 
three horizons. Beginning with the lowest, it is somewhat 
remarkable that a very large proportion of the Pharetrones in 
England has been yielded by the Lower-Greensand strata of 
Faringdon, in Berkshire. So abundant are these organisms 
that the bed containing them is known as the ‘‘Sponge-gravel.” 
It is a conglomerate of siliceous sand and gravel, hardened by 
a ferruginous cement. ‘The sponges form a large part of the 
bed; the material surrounding and envelopmg them, if we 
except the remains of other organisms, is entirely siliceous, 
while the sponges themselves are entirely calcareous and are 
acknowledged to have grown in the same beds in which they 
are now found. It is therefore altogether improbable that the 
sponges in this gravel-bed should have had their skeletons, if 
originally siliceous, so.completely replaced by calcite, a mate- 
rial which is now not present in these beds save in the remains 
of these and other calcareous organisms. On the other hand, 
the unaltered appearance of these sponges and the resemblance 
of the calcite of their skeletons to that of the shells and 
other organisms associated with them favour strongly the 
opinion that they still possess their original calcitic structure. 
No undoubted siliceous sponges have, so far as I am aware, 
been discovered with the Pharetrones in this sponge-gravel. 
Prof. Sollas admits the difficulty of explaining, on his hypo- 
thesis of the original siliceous composition of these sponges, 
their present calcareous condition in this bed, on account of 
not finding the silica resulting from the solution of their ori- 
ginal structure; but he supposes that it has been carried away 
by current-action of the seas and subsequent drainage of per- 
colating waters *. He makes no reference to what appears to 
* Ann, & Mag. Nat. Hist. 1878, vol. ii. p. 362. 
