DR. G. J. HINDE ON RECEPTACULITIDZ. 809 
shells of certain mollusca. The facts brought forward by Gimbel 
do not, however, appear to me to be sufficient to prove the organic 
nature of this fibrous crystalline structure. The constant direction 
of the radiation of the fibres may be attributed to the fact that the 
vertical axis of the spicular ray is the centre from which the rays 
diverge to the surface of the spicule. Thatthe faint concentric and 
parallel lines, noted by Giimbel in vertical and transverse sections of 
the spicules, may indicate their mode of growth by the addition of 
concentric layers seems extremely probable, but such markings 
might yet be shown even on the supposition that calcite had replaced 
silica, and so far from being directly opposed to the supposed 
relationship to Sponges, as Giimbel asserts, they are, in fact, strong 
evidences in favour thereof, since the spicules alike of calcareous 
and siliceous sponges are built up of concentric layers deposited 
round a central axial canal. 
If we now take into consideration the supposition that silica has 
been the original mineral constituent of the Receptaculitide, we are 
confronted by the fact that the instances are comparatively few in 
which this mineral now forms the skeleton, and even then itis, like 
the crystalline calcite, in a secondary condition. But from the 
experiences obtained of the changes which the organic silica of 
sponges undergoes in the fossil condition, there are reasonable grounds 
for believing that the skeleton of Receptaculites may have been of 
a siliceous and not of a calcareous character. Nothing is more 
common than to find the spicular mesh-work of undoubted siliceous 
sponges in the Jurassic limestones of Germany and elsewhere wholly 
replaced by crystalline calcite, but so that the perfect form is retained, 
and in some instances even the central axial canal can be detected. 
Equally familiar too is the fact that in the Upper Chalk of this 
country the siliceous skeleton of the sponges has been wholly 
removed by solution, and the cavities are now either empty or 
replaced by iron peroxide. And this solution and replacement 
are so general that in the sponge-beds of certain Jurassic areas 
it is very rare to find a single individual sponge in which the original 
siliceous skeleton is intact; whilst in the Upper Chalk of this country 
we must resort to the sponges enclosed in flint nodules to find a 
specimen in which the spicules are of silica, and even then the silica 
is in a secondary condition. The examples of undoubted siliceous 
sponges from Paleozoic rocks are not very numerous, but in many 
of them the skeletons have been either removed, leaving empty casts, 
or replaced by crystalline calcite, iron peroxide, and iron pyrites. 
The siliceous sponges of the genera Azlocopium and Astylospongia, 
from Silurian strata of the Baltic basin and in West Tennessee, are, 
indeed, often in a siliceous matrix; but it happens that the minute 
structure of the sponges themselves, when it is preserved at all, is 
generally a compound of iron or merely represented by empty moulds, 
and these sponges may be considered somewhat in the light of 
siliceous nodules in which the silica liberated from their own skeletons 
and those of other sponges has been aggregated. Other siliceous 
sponges of the genus Astrwospongia, from the Silurian of Tennessee 
