71, 



BRITISH FOSSIL SPONGES. 



4. Hexactinellid Spicules. — The typical spicule of this group lias 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. (5, 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, 



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A 



1 lit 



xiyo 



¥ Jty 



: x;c 





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, (b) 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 rays are 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 Coelopti/ckium 

 agaricoides, Goldf., showing the lantern or octahedral nodes, (g) Plumose flesh-spicule. (h) 

 Amphidisc 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, 1 c), and this subdivision may be carried to 



