— 6o — 



differed from the other specimens of that species and closely 

 corresponded with the Horstead spicules both in the mesh and 

 surface spicules. I think that this difference is sufficient to 

 indicate that these annulated spicules belong to another 

 species which I propose to name R. annulata. 



The mesh spicules of R. annulata are rare in the Horstead 

 material, whilst the surface spicules are very abundant. 



Racodiscula and allied Genera. 



(Plate IV, f5gs. 31—34. Pliite V, figs. 5—8.) 



Spicules of the mesh consisting of four equal or nearly 

 equal smooth arms radiating from a common centre; the ex- 

 tremity of the arms divided up into a mass of small twig- 

 like extensions which are united with the similarly formed 

 extremities of adjacent spicules. Length of spicular arms in 

 different spicules varying between 0,27 mm. and 0,45 mm.; and 

 between 0,045 '^m. and 0,067 nini- ^^ thickness. Surface 

 spicules consisting of horizontally expanded, numerously 

 branched delicate heads with short rudimentary shafts. The 

 width of the heads varies between 0,517mm. and 1,17mm. 



The mesh spicules of Racodiscula and allied genera are 

 very minute in comparison with those of Plinthosella and the 

 slender arms have been so much eroded by fossilization that, 

 notwithstanding they are present in considerable numbers it 

 is difficult to find good specimens. In many examples the 

 spicular arms are reduced to slender threads and their termi- 

 nations only show the stumpy ends of the fibres which formed 

 the relatively large cushion-like mass by which they were 

 united together. There are .several genera of sponges, whose 

 mesh-spicules, with the exception of slight variations in the 

 length of the rays and in the size of the intricate knot at 

 their point of union, are closely similar to each other. This 

 has been shown by Zittel in the spicules of the fossil sponges 



