30 DR. J. S. BOWERBANK ON HYALONEMA MIRABILE. [Jail. 10, 



two together ; but occasionally they occur in groups of ten or twelve, 

 without any approach to a definite arrangement (PI. V. fig. 1). 

 These spicula are comparatively large and stout. They have eight 

 rays at each end of the shaft, the two groups of radii curving towards 

 each other to such an extent that each forms the half of a regular 

 oval figure, the opposite apices being separated to the extent of about 

 the length of one of the radii. Each ray is in form like a double- 

 edged obtusely pointed knife bent near the handle in the direction of 

 a line at right angles to the inner surface of one of its flat sides ; 

 and each ray is strengthened and connected with the shaft of the 

 spiculum by a stout curved web of silex, which extends from a little 

 below the inner surface of the ray to a point on the shaft about op- 

 posite to its middle. The edges of each ray are also slightly curved 

 inward (PI. V. fig. 2). The smaller or secondary system of birotu- 

 late spicula differ somewhat from the larger ones in structure. 

 They are not fimbriated at the base, as those of the larger ones are, 

 nor have the radii tlie same distinct cultelliform figure (PI. V. fig. 3). 

 Their position in the s])onge is also different. The larger ones are 

 always irregularly dispersed ; while those of the smaller system are 

 usually congregated in considerable numbers around the large ske- 

 leton-fasciculi, their direction being coincident with the axial line of 

 the fasciculus (PL V. fig. 4) ; a few, compai'atively, are dispersed, 

 but this mode of position a])pears to be rather the exception than 

 the rule. The shaft is cylindrical, and has short stout tubercles 

 dispersed over all its parts, and the radii are so long in their pro- 

 portions that the opposing apices very nearly toiich each other. 



The third form of interstitial spiculum, the cylindro-cruciforra 

 one (PI. V, figs. 10, 11, 12, 13, 14), appears to appertain more espe- 

 cially to the cloacal system ; they are found abundantly dispersed 

 near the inner surface of the coriaceous dermis of the spiral column 

 of the sponge ; but they occur in by far the greatest number between 

 the basal portion of the spicvda of the spiral column, and in their 

 immediate neiglibourhood, intermixed with the large hexradiate spi- 

 cula of the interstitial tissue of the great basal mass of the sponge ; 

 and at the top of this m.ass the spiral column is surrounded by a 

 profusion of them. 



The radii are short and very stout in their proportions, their length 

 varying from twice to five or six times their own diameter ; and the 

 four rays are frequently of differoit lengths. They are profusely 

 covered with large, stout, more or less conical spines, and especially 

 so at their distal extremities. In all these characters they vary to 

 a considerable extent even in the same group. They appear to he 

 more matured in the basal portions of the sponge than in the coria- 

 ceous dermis of the spiral column ;• in the latter position they are 

 frequently represented by short, stout, entirely spined cylindrical 

 spicula (PI. V. fig. 10) ; Ijut between this rudimentary state and the 

 completely cruciform spiculum specimens may be found in every in- 

 termediate stage of development. Occasionally a spiculum may be 

 found with a filth ray, indicating that the cruciform spiculum is in 

 truth only a modification of the regular hexradiate type of the inter- 



