488 Proceedings of the Royal Irish Academy. 



branched and terminate in expanded ends, whicli frequently present 

 an almost absurd resemblance to a foot or boot ; the margin of this 

 terminal part is fi'equently extended into a lamellar expansion or 

 foliation. Though usually smooth, or sparsely spined, the rays may 

 sometimes be richly tuberculated. The remaining upper part of the 

 centrum sprouts out into a profusion of spines, which by branching, 

 and emitting twig-like smaller spiaes, with rounded ends, give to the 

 head of the corpuscle a characteristic shrub-like appearance. The 

 body of the centrum may also bear tubercles and short, simple spines, 

 though sometimes it remains smooth. 



When by prolonged boiling in potash the ends of the rays have 

 been dissolved to sharpened points, and the coi'puscle has, in other 

 respects, become simplified, it presents a striking resemblance to the 

 spicules of Holasterella, a fossil sponge of the carboniferous lime- 

 stone, described by Carter. 



The second kind of spicule (type 2) is devoid of the antler-like 

 spines, and possesses a larger number of rays (from six to eight) ; 

 whence we may conclude that the spines just mentioned represent 

 stunted rays. The rays may be smooth, but are more usually covered 

 with short spines having rounded ends. 



The third kind of corpuscle differs more considerably from the 

 type spicule : the centrum loses its somewhat spherical form, and 

 flattens out into a plate, which extends at the margin into rays 

 flattened in the same plane as itself. These rays are shorter, often 

 more irregular, and less clearly defijied from the centrum than ia the 

 other kinds of corpuscles. Sometimes one or more rays are given off 

 at light angles to the surface of the centrum, which, together with 

 the rays, is usually richly ornamented with rounded tubercles. 



"Within the centrum, placed more or less excentrically, is an oval, 

 finely granular space, one of the granules with a spherical contour 

 being much larger than the rest, and the whole presenting a striking 

 resemblance to the nucleus of a cell with its nucleolus : as such, or at 

 least as part of a cell, it is, indeed, regarded by Schmidt. That it is 

 an indication of a cell I think very probable, but not that it is actually 

 a nucleus. In the first place it is completely enclosed in the siliceous 

 centrum, having no communication with the exterior. The axes of the 

 rays have also a granular character, but are not so densely crowded 

 with granules as the oval space. The granules of the rays extend 

 into the centrum, and are arranged within it in curves more or less 

 concentric with the oval space, without, however, coming into dii'ect 

 connexion with it : on the contrary, the substance bordering this is 

 always clear and homogeneous, like glass. 



Observed with oblique light, the granules which, with transmitted 

 light, are dark-brown in colour, or even black, present a white and 

 glistening appearance, strongly suggestive of air cavities. That they 

 are such is, I think, shown by the fact that when on boiling with, 

 potash the solvent gains access to these spaces, they soon become 

 hollowed out, and an oval cavity replaces the granular ellipsoid. 



