30 



bous heads , but which I now regard as trifids in which the 

 head rays have been broken off (Plate I, fig. 4), the canal is 

 now extremely wide ; but in the majority of the specimens 

 the canal is partially or wholly infiltrated with silica. The 

 surface of these spicules is very rough, and the peculiar 

 radiate structure of the silica of the interior is well displayed. 

 Scarcely any of these spicules are complete, the lower part 

 of the shaft being mostly absent. A small specimen with 

 the shaft complete has a length of 7 mm.; others in which 

 the terminal portion is wanting, measure 8.77 mm. long, 

 with a width of the bulbous portion of the shaft of 

 0,585 mm. They are not uncommon. 



The gradual development of the rays of the head is 

 well shown in different examples of these spicules. In one 

 instance there are but two of these knob shaped rays, as if 

 the summit of the shaft had been merely bifurcated (fig. i) ; 

 then follow specimens in which three simple knobs or rays 

 are displayed (fig. 2), other specimens have one or more of 

 these knobs but slightly constricted (fig. 3) whilst in others, 

 the bifurcation of the rays is carried still further , the primary 

 rays are somewhat compressed and flattened, and each extended 

 into two short, obtusely pointed arms (fig. 4). The numerous 

 transition forms would seem to indicate that these variously 

 headed spicules may all have belonged to the same species 

 of sponge. 



I am unable to find any description of fossil or existing 

 sponges with similar spicules; their form would indicate that 

 they were probably the 'zone' spicules of a sponge like Geodia, 

 and as they are distinct both in size and in the peculiar knob- 

 like character ot the head rays, I propose to designate them 

 Geodia ? clavata. Spicules with a constricted neck, but with a 

 somewhat different form of the head rays, and besides of much 

 smaller dimensions are figured from the North of Ireland. 

 (Wright: op. cit. pi. II fig. 14). 



