1867.] DR. J. S. BOWERBANK ON HYALOXEMA MIRABILE. 29 



form, and are of a complicated and very interesting structure. They 

 consist of a short stout cruciform base, with a long spicular ray as- 

 cendingly and entirely spinous, projected at right angles from the 

 centre of the basal radii. The spines on the spicular ray are similar 

 in form and mode of disposition to those of the external defensive 

 spicula, but very much longer in proportion to the size of the spicu- 

 lum, frequently exceeding in their length the diameter of the shaft 

 on which they are based (PI. V. fig. G, and fig. 1 b in situ). 



The radii of the cruciform bases are also slightly spiculated towards 

 their apices. They are thickly distributed on the fasciculi of the 

 skeleton, and frequently equally so on one side of the interstitial 

 membranes, probably that which forms the surfaces of the interstitial 

 spaces, and they are especially abundant near the exterior of the 

 sponge. 



The four basal radii appear firmly cemented to the membrane — 

 but not immersed in its substance, as they do not appear to leave 

 their impression when removed from it, nor do they bring any por- 

 tion of the membrane away with them. 



In some part of the tissues these spicula are very much modified 

 in form. In ordinary cases we find the basal radii short and stout, 

 and not more than a fourth or a fifth of the length of the spicular 

 ray ; while in other cases the basal rays aie very nearly as Ions as 

 the spicular one, the only difference in their structure being that 

 the latter is very strongly spinous, while the former have the spines 

 comparatively very slightly produced. 



The interstitial membranes, when not covered with spiculated cru- 

 ciform spicula, are often abundantly furnished v.'ith long slender 

 flexuous acerate tension spicula, with a central inflation indicative of 

 their being an incipient condition of either rectangulated hexradiate 

 or rectangulated triradiate forms ; and the latter one occasionally is 

 found among them (PI. V. fig. 7). 



The interstitial spicula of this sponge are very numerous, and ex- 

 ceedingly various in size .nnd form. They are of three very dis- 

 tinct descriptions: — first, rectangulated hexradiate, large and small 

 (PI. V. figs. 8, 9) ; second, fimbriated multihamate birotulate (PI. V. 

 iigs. 2, 3); and third, cvlindro-cruciform (PI. V. figs. 10, 11, 12, 

 13, 14). 



The first of tb.ese forms abound immediately beneath the apparent 

 line of the dermal membrane in the large basal mass of the sponge ; 

 the greater portion of them are large, and they are disposed with a 

 considerable approach to regularity, and amongst them there are 

 frequently groups of the smaller variety of this form (PI. V. fig. 8). 

 They are also rather abundant near the basal portion of the spiral 

 column of the cloacal system of the sponge, ancl they are found more 

 sparingly dispersed in all parts of the basal mass. Generally speak- 

 ing the whole of the six radii are fully produced ; but occasionally 

 pentradiate forms are found. 



The second form or fimbriated multihamate birotulate spicula are 

 generally found dispersed amid the interstitial tissues of the large 

 basal mass of the sponge. There are usually not more than one or 



