82 BRITISH FOSSIL SPONGES. 



spicules (Fig. 4, b). As these facets are very numerous, and radiate in all direc- 

 tions from the main axis of the spicules, they form by their union an extremely 

 intricate mesh work with minute irregular interspaces. In some instances the 

 spicules thus united produce anastomosing fibres with open interspaces, in which 

 the circulation is carried on, as, for example, in Pachinion, Zitt. ; in others, as in 

 Seliscothon, Zitt., they form thin vertical lamellae. 



The spicules of the dermal layer in lithistid Sponges are arranged so that the 

 small vertical ray penetrates into the wall of the Sponge, whilst the horizontally 

 extended head-rays cover the outer surface, frequently overlapping each other, 

 thus forming a smooth covering with only microscopic interspaces. They are not 

 organically attached to each other, or to the skeletal spicules, and it is very 

 seldom that they have been preserved in situ in the fossil state. In some cases 

 the dermal spicules appear to have extended over the cloacal surface as well as 

 over the exterior of the Sponge. In the genus Dory derma, the trifid spicules of the 

 dermal layer are tightly packed, with their shafts parallel to each other, into the open 

 meshes of the skeletal-spicules, and their head-rays project, like arrows in a quiver. 



Hexactinellida. — In this division the six-rayed spicules are disposed so that 

 their rays overlap each other, and produce a framework which, when regularly 

 developed, has quadrate or subquadrate boundaries. The spicules may be simply 

 held in position by the sarcode or fleshy portion of the Sponge, or they are united 

 organically together by a common siliceous sheath. Fossil examples of those 

 Sponges in which the spicules are not organically fused together are very rare, 

 though the detached spicules are extremely abundant. Judging by existing forms 

 of these Lyssakine Sponges, the spicules are generally arranged into elongate, 

 loose fasciculate fibres or open tissues, which in some genera, as in Euplectella, for 

 example, cross each other in regular lines. Not strictly in all cases are the 

 spicules free from each other, for they are sometimes soldered together laterally, 

 and at the crossing of some of the larger rays, by a deposit of silica. In many 

 recent Lyssakine Sponges the spicules of the dermal layer have the same regular 

 arrangement as those of Dictyonine Sponges, and form a meshwork with quadrate 

 interspaces, but the spicules are not fused together. A fragmentary example of 

 this is shown in Hyalostelia Smithii (PI. VI, fig. 1). The elongated spicules of 

 the anchoring rope of this group of Sponges are disposed either parallel to each 

 other, so as to form rounded or compressed fascicles (PL I, fig. 3 ; PL VI, figs. 2, 

 3 d), or they extend singly through the rock. Though the component spicules of 

 these ropes are not organically attached together, yet they are found in close 

 contact with each other for considerable distances, and this undisturbed arrange- 

 ment may probably be owing to the circumstance that these anchoring ropes 

 were embedded in the mud during the life of the Sponge, and were thus preserved 

 from disintegration after its death. 



