The Skeleton and Classification of Calcareous Sponges. 65 



shows the last rudiments of the spicule of G. coriacea slowly 

 decalcified by the prolonged action of glycerin. Fig. 5 gives decal- 



Fm. 5. Spicule of G. coriacea ( x 1500), showing sculpturing by decalcification in 

 Canada balsam. One ray is fractured and decalcified, the other two con- 

 ventionally truncated. 



FIG. 6. Young spicules (xlOOO). a of C. clathrus, b (an equiangular spicule in 

 perspective) of A. cerebrum, c of L. Lielerkuhnii. 



cification figures on a spicule belon ging to the same sponge under the 

 action of Canada balsam, and fig. 4, spicules of C. clathrus attacked by 

 glycerin. They appear to be generalisable, like those drawn and 

 explained by Ebner for Leucaltis solida, into triangles whose angles are 

 on the external face of the spicule opposite the rays, and intermediate 

 to them 011 the gastral face ; complementary, as are the edges on the 

 opposing poles of a rhombohedron : the crystallisation is therefore 

 closely comparable with that of Schiefer spar (a natural form of 

 calcite crystallising in flat hexagonal plates), and still more with that 

 of a specimen of dolomite (calcite with a little magnesium), No. 427 

 in the Cambridge museum, which is crystallised in flat triangular 

 plates. The rays of the spicule are opposite the faces of the rhombo- 

 hedron on the gastral surface, opposite the angles of the rhombohe- 

 dron on what may be called the " dorsal " surface.* 



* [The initial etchings by Canada balsam on the dorsal surface of the rays of 

 G. coriacea frequently appear under the 2 mm. immersion lens to be small 

 rhomboids, orientated so that lines bisecting their acute angles are parallel with the 

 morphological axis of the ray. In some spicules of C. clathrus prolonged decalci- 

 fieation by glycerin yielded internal cavities of similar outline. The optic axis, 



