186 Dr. G. J. Hinde on Fossil Calcispongie. 
fossil sponges which occur in strata from the Devonian to the 
Cretaceous inclusive. In regard to outer form, these sponges 
possess the same variety which is met with in fossil siliceous 
sponges ; but as a rule they are individually of much smaller 
dimensions. Their main characteristic feature is the possession 
of a fibrous skeleton, composed of carbonate of lime, and, un- 
like many other fossil sponges sometimes associated with 
them in the same strata, whose skeletons are also calcareous, 
this material forms the original skeleton, and is not the result 
of subsequent replacement. The fact, now generally recog- 
nized by those who have studied fossil sponges, that the 
present calcareous or siliceous condition of these bodies 
affords of itself no certain indications of their original compo- 
sition, has caused a great amount of confusion. It is only 
within a comparatively recent period that the instability of 
the organic silica in sponge-spicules and its replacement by 
calcite in many fossil sponges has been demonstrated. Before 
this fact became known all fossil sponges with calcareous 
skeletons were regarded as possessing this mineral constitu- 
tion originally ; for calcite was generally thought to be much 
less stable than silica, and it was believed impossible that it 
could replace this latter material. At the present time, with 
many authorities, the belief is in the opposite direction, and, 
instead of regarding all sponges with calcareous skeletons as 
formed originally of this material, it is asserted that no known 
fossil sponges were originally calcareous, and that all those 
with calcareous skeletons have become so by dissolution and 
removal of the original silica and its replacement by calcite. 
It becomes therefore a matter for the exercise of judgment to 
determine the original composition of any fossil sponge. In 
the case of sponges, for example, whose skeletons are built up 
of lithistid or hexactinellid spicules we are enabled at once to 
determine that their composition, though now frequently cal- 
careous, was originally siliceous; for we know that in all 
existing sponges with this type of spicular structure the com- 
position is invariably siliceous; and, further, the fossil ex- 
amples occur frequently both in a calcareous and siliceous 
condition, and those which are siliceous exhibit the outer 
form and interior canal-structure of the component spicules in 
a comparatively perfect state, whilst the opposite occurs with 
those which are calcareous; and from this we may conclude 
that this latter material is not the original constituent of the 
skeleton of these sponges. 
The fibrous character of the skeleton of the Pharetrones, 
however, is so unlike that of any existing calcareous sponge 
that we are unable to apply the comparison which enables us 
