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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 RK. annulata are rare in the Horstead 
material, whilst the surface spicules are very abundant. 
Racodiscula and allied Genera. 
(Plate IV, figs. 31—34. Plate V, figs. 5—S8.) 
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 0,27mm. and 0,45 mm.; and 
between 0,045mm. and 0,067 mm. in 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 /ossz/ sponges 
