356 



Mr. W. J. SoUas on the Genus Catagma. 



The longitudinal sections of the spicules generally exhibit a 

 number of opaque dots, usually irregularly dispersed ; so that 

 one of them occurring in a transverse section would simiilate 

 an axial canal cut through. This makes one less sure that 

 axial canals really exist in these spicules ; but since the dots 

 sometimes take a linear arrangement in the longitudinal sec- 

 tions (PI. XIV. fig. 16), they may, after all, indicate axial 

 canals which have been partially filled up. 



Relative Position in the Fibre of the two kinds of Spicules. 



The filiform spicules chiefly occur in the outer part of the 

 fibre, often forming the exterior third on each side, though 

 sometimes less and sometimes, on the other hand, more of it, 

 in some cases apparently occupying nearly the whole of the 

 fibre almost to the exclusion of the other kind of spicules. 



Fig. 1. 



Section (partly diagrammatic) across fibres near the external surface of 

 Catagma macroporics. ( X 70.) 



a, uniaxial spicules ; b, transverse section of a multiradiate ; c, echinating 

 multiradiate spicules ; d, interspace of a mesh, lined with granular 

 material and filled up with calcitic crystallization. 



The multiradiate spicules are usually axial in position, forming 

 a core which is about one third the diameter of the whole fibre, 

 though sometimes wider. This core looks, at first sight, like 

 a congeries of irregular calcite crystals ; but upon very careful 

 analysis by the microscope it is seen to consist of circular and 



