SoLLAS— 0;?- Sponge Spicules. 387 



fragments, such an outline. That they are not cylindrical may 

 easily be proved by rolling them under a cover-glass, and measur- 

 ing their diameter in different positions. Their optical double- 

 sidedness, due to difference in refractive index, is also then clearly 

 seen without the aid of polarized light. 



HseckeP has recognised the fact that the section of calcareous 

 spicules is not always circular, since he says that " in the acerates, 

 as well as in the triradiates and quadriradiates, the transverse sec- 

 tion of the ray is usually circular, seldom elliptical, and much 

 seldomer linear, so that the ray is flattened band-like." He gives 

 no instances however. 



Application to the Pharetrones Question. — Since the acerate spi- 

 cules of some calcisponges are oval in section, or possibly rhom- 

 boidal, this enables us, when other characters are wanting, to 

 distinguish such spicules from the similar ones of the siliceous 

 sponges ; and if the Pharetrones were originally calcareous, their 

 colossal spicules should afford us oval and rhomboidal sections ; for 

 even if the rhomboidal outline observed in the recent spicule were 

 due to etching, it is also true that the fossil spicules have suffered 

 from the same cause, and so should exhibit similar outlines. On 

 turning to my thin slices of Pharetrones I find that a rounded 

 polygonal outline frequently characterises the sections of colossal 

 spicules ; many offer an oval section ; and though, of course, a 

 cylindrical spicule will give an oval outline if cut obliquely, yet 

 in many cases the sides of the ellipse in the fossil spicules are 

 far too flattened to be explained in such a manner (pi. xv., 

 fig. 12). Though oval and polygonal outlines are common, cir- 

 cular ones are not absent ; but that does not affect the argument, 

 as we do not assert that cylindrical spicules do not occur among calci- 

 sponges, but that oval and polygonal ones are not known amongst 

 the siliceous sponges.^ On turning to the illustrations in my Paper 

 on Catagma, I find that I had at that time represented the sections 

 of these large spicules as somewhat polygonal, though I did not re- 

 cognise the fact at the time, nor if I had should I have understood 

 its significance. A distinct polygonal outline is given to the rays 

 of many calcisponges in Hseckel's '' Monograph," v., Taf. 27, &o. 



^ Hseckel, "Die Kalkschwamme," Band i. p. 203. 

 TKe shafts of Lithistid skeletal corpuscles are, however, not always circular in 



section 



