SKELETAL STRUCTURES. (ey) 
stoma! from the Cenomanian of Essen, and I have also observed it in detached 
spicules of Lewconia from the Phocene beds of St. Erth. 
3. Four-rayed spicules. These may be described as three-rayed spicules in 
which an additional ray radiates from the point of junction of the three rays either 
at right angles or obliquely. This fourth, or apical ray as it has been termed by 
Haeckel, is, in the fossil forms in which I have noticed it, frequently shorter than 
the three facial rays of the spicule (Fig. 7, &). Four-rayed spicules are present 
in the dermal layer of Tremacystia D’ Orbiynyi,’ Hinde, and possibly in the dermal 
layer of other fossil calcisponges as well, but unless the spicules can be isolated it 
is difficult to determine whether a fourth ray is present or not. Four-rayed 
spicules are also present in the fibres of Sestrostomella clavata,> Hinde; in these 
the rays are curved. 
Tue DispositIoN OF THE SPICULES IN THE SKELETON. 
The manner in which the elementary spicules, whose forms have just been 
described, are combined together to form the skeleton of the Sponge is very varied 
in the different groups. According to the nature of this union so is the capacity 
of the Sponge to resist the disorganizing influences of fossilization, and it probably 
explains the rarity in the fossil state of certain groups of Sponges which are 
extremely abundant in the present seas. We proceed to consider first the skeleton 
of siliceous Sponges. 
Monactinellide and Tetractinellide. In these two groups, which form the large 
majority of existing siliceous Sponges, the spicular elements of the skeleton are 
not organically fused together, but are held in their natural positions by an 
envelopment of a horny substance known as spongin. In some Sponges this 
connecting substance is reduced to a small amount, which merely surrounds 
the terminal ends of the spicules, whilst in others the spicules are completely 
enveloped by it, and thus held together so as to form a meshwork of fibres, 
in which they are arranged parallel with each other, or they may be grouped 
in bundles which branch and anastomose, or radiate from the base to the 
summit of the Sponge. As this connecting horny substance inevitably decays on 
the death of the Sponge, the spicules become detached and fall apart, and only 
under very exceptional conditions of preservation does the skeleton retain its 
natural form in the fossil state. As a matter of fact, entire Sponges of these 
groups, or even connected fragments of the skeleton, are of the rarest occurrence, 
PD Op.cit., ps 12: 
2 «Ann. and Mag. Nat. Hist.,’ 8. 5, vol. x, p. 192, pl. xi, figs. 1—8. 
3 Id., pl. xii, fig. 16. 
