a Gis 
fect as in recent spicules. | Another feature of the little 
change which these spicules have suffered, is shown by their 
being neutral to polarised light, thus proving that the silica 
still retains its colloidal condition, although it has lost that 
beautiful clear glassy appearance which characterizes the spic- 
ules of recent sponges. The state of preservation in which 
these Coesfeld spicules are found may, however, be regarded 
as exceptional, in the majority of instances in which detached 
spicules have been noticed the silica appears to be now in the 
form of Chalcedony. 
As I have already mentioned, the eroded condition of 
the larger spicules plainly indicates that none of those much 
smaller forms which have been termed flesh-spicules, have 
stood any chance of being preserved, and therefore, for the pur. 
poses of classification, the characters of the skeleton spicules 
will alone have to be relied upon. Of these I separated 
some thousands of specimens and they present the most ma- 
nifold varieties both of form and size. In dimensions they 
vary between 0.18 mm., the diameter of small globo-stellates, 
and the comparatively great dimensions of 8.77mm., the length 
of the longest trifid spicules. The various forms comprise straight 
and curved cylindrical acerates, conical and fusiform acuates, a 
great variety of trifid spicules (i. e. those with an elongated shaft 
and a head of three rays); quadrifid spicules, globates and 
stello-globates, contorted skeleton spicules of Lithistids and 
free and combined six-rayed spicules. Besides these different 
forms of isolated spicules there are a few specimens in which 
small fragments of the mesh-work of the sponge, showing the 
spicules naturally attached together, have been preserved. A 
glance at the figures given in the different plates will better 
enable an idea to be formed of the great diversity presented 
by these spicules, mixed up together in the interior ot 
this single flint, than can be conveyed by a mere verbal de- 
scription. 
