74 BRITISH FOSSIL SPONGES. 
4, Hexactinetti Spicutes.—The typical spicule of this group has six equal rays 
radiating from a common centre, at right angles to each other, thus with three 
equal axes. Modifications of the type form occur through the unequal develop- 
ment of one or more of'the individual rays (Fig. 6, b), which may be either 
extended or reduced, or even aborted altogether. Thus spicules frequently occur 
in which one ray is absent, so that a nail-shaped form results, with four rays in 
one plane, and the unpaired ray at right angles to it (Fig. 6,c). In other cases, 
Fig. 6.—Different forms of fossil hexactinellid spicules and fragments of spicular mesh. (a) Spicule 
with lantern or octahedral node, showing the extension and union of the canals of the different 
rays in the centre of the node. (4) Simple detached spicule with rays of unequal length and 
slightly inflated compact node, Hyalostelia fusiformis, Hinde. (¢c) Detached nail-shaped spicule 
in which only five raysare present. (d) Spicule of Receptaculites occidentalis, Salt. (e) A frag- 
ment of the skeletal mesh of Sestrodictyon convolutum, showing the compact nodes and the apparent 
continuity of the canals between the nodes. (f) A fragment of the skeletal mesh of Celoptychium 
agaricoides, Goldf., showing the lantern or octahedral nodes. (g) Plumose flesh-spicule. (A) 
Amphidise flesh-spicule. 
the vertical ray of the spicule is reduced to a rounded stump, as in some of the 
spicules of Hyalostelia Smithii (Pl. VI, figs. 1,1, a). By the reduction of one axis 
the spicule becomes cruciform, as in Protospongia (Pl. I, figs. 1, a, 2, a). The 
reduction of the number of rays may even proceed so far as to leave a single 
elongated rod or fusiform axis, not to be distinguished externally from a spicule of the 
monactinellid type, but its real character is shown by the occasional slight develop- 
ment, in one portion of its length, of the transverse axial canals. As a general rule 
the spicular rays are simple and smooth (Fig. 6, b), but in some cases they bifurcate 
as in Spiractinella (Pl. VIII, figs. 1, 1c), and this subdivision may be carried to 
