Dr. G. J. Hinde on Fossil Calcispongie. 189 
me the far greater difficulty of accounting for the complete 
substitution of silica by calcite in a stratum where free calca- 
reous material does not exist. 
Passing now to the Upper Greensand, Pharetrones occur, 
though not very abundantly, in beds of this age at Warmin- 
ster ; and in the same locality and in beds of similar lithological 
characters there are numerous examples of siliceous lithistid 
sponges. The comparison of examples of these different 
groups thus preserved under similar conditions is very instruc- 
tive, and shows, one may say almost at a glance, such great 
differences, that no one could suppose that both these groups 
were originally siliceous in composition. The matrix is of 
quartzitic and green grains, hardened by a siliceous cement, 
without any trace of calcareous material. The Pharetrones in 
this Warminster Greensand are entirely calcareous; their 
outer form is perfectly preserved; and the fibre, both at the 
surface and in the interior of the sponge, is alike of a compa- 
ratively soft white earthy material, whilst the interspaces are 
filled by the transparent quartzitic and green grains of the 
matrix. Altogether different are the appearances presented 
by the siliceous lithistid sponges in these same beds. The 
sponges themselves are siliceous ; the outer form is preserved ; 
but the exterior surface in the majority of the specimens ex- 
hibits only the cemented grains of the matrix, with here and 
there partially obliterated spicular structure. In the interior 
the spicular skeleton is either imbedded in glassy silica, or it 
not unusually happens that the siliceous spicules have been 
dissolved and their empty moulds remain. In no single in- 
stance have I found these moulds refilled with calcite. On the 
hypothesis of the siliceous origin of the Pharetrones it appears 
inexplicable that they should now be exclusively and com- 
pletely calcareous, whilst the undoubtedly siliceous Lithistids 
should yet retain their siliceous structure. ‘The present diffe- 
rences can only be understood by admitting the calcareous 
origin of the Pharetrones. 
It is, however, on the Pharetrones in the Cambridge Green- 
sand, and more particularly on the structure of Pharetro- 
spongia Strahani, Sollas, that Prof. Sollas bases his argu- 
ments in favour of the siliceous character of these fibrous 
sponges. ‘The matrix enclosing the sponges is a dark bitu- 
minous material; the brown and greenish grains of which it is 
composed appear to be cemented, in part at least, by calcite ; 
for after treatment with acid they are disintegrated. The 
sponge-fibres are mainly composed of calcite which readily 
dissolves in acid ; but very frequently there remains here and 
there a thin delicate film, forming, as it were, a sheathing on 
