SKELETAL STRUCTURES. 65 
together in different ways, so as either to form a continuous framework or loose 
open fibres, which serve as a support to the soft fleshy portions of the Sponge. 
These mineral particles are termed spicules, a word used in a general sense to 
indicate the individual element or particle of the skeleton, whatever may be its 
form. ‘The spicules of fossil Sponges, like those of recent ones, are composed 
either of silica or of carbonate of lime. In all essential features they resemble the 
spicules of existing Sponges, and it may be assumed that like these latter they have 
been formed in cells in the mesoderm of the Sponge. Spicules consist of an outer 
wall of concentric layers, enclosing in most, if not in all cases, a delicate longitu- 
dinal axial canal, which extends either partially or entirely the length of the spicule, 
and into each of its main arms or rays. This axial canal is usually much wider in 
fossil spicules than in recent forms ; in the living condition the canal appears to 
have been filled with protoplasm, but in the fossil state it is usually occupied by a 
mineral substance of the same general character as the spicular wall. If the in- 
filling material has the same optical characters as the wall of the spicule, the canal 
can no longer be distinguished, and the spicule appears solid throughout; but not 
unfrequently the infilling substance of the canal is more resistent than the spicular 
wall, and remains even when this latter has been dissolved away. Between each 
of the concentric layers of the spicular wall there appears to have been originally 
a thin film of organic substance. The structural layers are but rarely seen in 
fossil spicules; they can, however, be occasionally distinguished in the rod-like 
spicules of Hyalostelia (Pl. VI, figs. 2 a, 2 ¢, 3 f). 
Two different kinds of spicules are usually recognised in recent Sponges, 
skeletal-spicules and flesh-spicules, and both kinds are present in the fossil 
forms, though the latter are of very subordinate importance in the fossil state, 
owing to their rare occurrence. The skeletal-spicules form the main body of the 
Sponge and are usually of much larger dimensions than the flesh-spicules. These 
latter, on the other hand, exhibit a much greater variety of form than the skeletal- 
spicules; they are not attached together, but merely held in position by the soft 
tissue of the Sponge, and consequently are not often found in their natural position 
in fossil Sponges. 
The two chief considerations in the study of fossil Sponges are, first, the 
character of the individual spicules, and next, the mode in which they are combined 
to form the skeleton. The skeletal-spicules of fossil Sponges have been arranged 
in groups based on the number and disposition of their axes or rays. Taking 
first those of siliceous Sponges, the following typical divisions may be enumerated : 
1. Monactinellid spicules, or those in which a single axis is present. 
2. Tetractinellid spicules, in which there are four rays or axes, which radiate 
from a common centre in the same direction as the axes of a regular, three-sided 
pyramid, 
