centre of the spicule, which is either formed by a thickening 
of the silica of each ray at the point of junction; or by the 
giving off of slight buttresses from each ray which unite 
together and form as it were a framework, of the figure of 
a hollow octahedron, to inclose the junction of the canals 
of each ray, in the centre of the spicule. To this latter 
structure the term «lantern» node has been applied. In addition 
to these spicules of the skeleton there are in the living 
examples of the order very minute ‘flesh’ spicules of wonder- 
ful variety and beauty of form, but in the fossil sponges 
these have not been preserved. The skeleton of the Hexac- 
tinellid sponges is either built up of spicules which are merely 
connected together by the sarcode or else the spicules are 
welded together by the extremity of their rays to form a 
connected mesh-work with regular oval or oblong interspaces. 
The hexactinellid sponges are also provided with an exterior 
surface layer, composed of a delicate perforated siliceous 
membrane which, in some instances at least, can be seen to 
be formed by a modification of spicules similar to those of 
the interior skeleton. In some species too the sponge was 
anchored to the sea bottom by elongated thread-like spicules 
terminated by small anchor-shaped heads. 
In this Horstead flint the hexactinellids are represented 
by free spicules of various sizes; by small portions of the 
spicular mesh of different species, also by fragments of the 
surface membrane and by portions of the root-fibre and the 
small terminal anchors. The free spicules appear to belong 
to sponges of the group of the Lyssakina in which the 
spicules were only connected by sarcode. Fragments of 
the mesh-work of the group of the Dictyonina are not 
infrequent and occur oftener than in the case of the Lithistid 
sponges, which may be accounted for by the fact that the skeletal 
mesh of the Hexactinellids is formed by a complete welding 
together by silica of the adjacent spicular arms, and not, as 
in the Lithistids, by a mere interlocking of their extremities. 
