SKELETAL STRUCTURES. 75 



such an extent that the spicule appears stellate (PI. VIII, figs. 1 g, 1 h). The rays 

 are not infrequently more or less covered with minute spines or even moniliform. 

 In the abnormal family of the Beceptaculitida, the distal or external ray of the 

 spicules is modified into a horizontally extended polygonal plate (PI. IV, fig. 2), 

 immediately beneath which are the four transverse rays (PI. IV, fig. 2, d), whilst 

 the proximal ray may be either greatly reduced, or normally tapering and pointed, 

 or connected with an internal plate as in the genus Beceptaculites itself. 



In the spicules which serve for anchoring the Sponge to the sea-bottom, one 

 ray is extremely elongated, reaching to a length of several inches, and at its distal 

 extremity the other four rays are developed in the form of small recurved hooks 

 (PL VI, figs. 2, e—Jt). 



The junction of the rays at the central node in hexactinellid spicules may be 

 formed by their simple union, or there may be a sub-spherical inflation, or from 

 each ray there are given off near the central node short processes or balks, 

 which pass obliquely to the proximate rays and form the outlines of a regular 

 octahedron (Fig. 6, a, f). The rays, in greatly diminished thickness, serving 

 merely as sheaths to the axial canals, are continued into the centre of this hollow 

 octahedron, where they unite together. This peculiar modification of the spicular 

 node was first noticed by Toulmin Smith in the Ventriculites from the Upper 

 Chalk ; it occurs also in the Jurassic genus Pachyteichisma, Zitt., and in many other 

 fossil genera, and also in the recent Myliusia, Gray. Such nodes are termed 

 octahedral or lantern, whilst the ordinary nodes are known as simple or compact. 



In all complete hexactinellid spicules there is an axial canal in each ray, and 

 the six branches unite together in the centre, both of the spicules with simple, and 

 those with octahedral nodes. The presence of these canals serves to indicate 

 individual spicules, even where a subsequent deposition of silica has completely 

 masked the form of the spicule by merging it in a common extended membrane, as 

 in the dermal layer of many Sponges of this group. 



In addition to the skeletal-spicules, the so-called flesh-spicules are largely 

 developed in recent hexactinellid Sponges, but, as already mentioned, their extreme 

 rarity in the fossil state renders them only of subordinate interest to the palaeon- 

 tologist. They are usually of far smaller proportions than the skeletal-spicules, 

 and exhibit a great variety of forms, which, however, in the majority of cases, 

 can be recoguised as modifications of the typical six-rayed spicule of the 

 skeleton. 



Two characteristic flesh-spicules recently discovered in pre-Miocene strata in 

 New Zealand are shown in Fig. 6. In one (g) known as " plumose " or " pinulus," 

 the four transverse rays are relatively small, whilst the fifth ray is unusually elon- 

 gated and is thickly covered with upward-projecting spinous processes. In the 

 other (//) or amphidisc spicule, a cylindrical rod has, at both ends, an umbrella- 



