Genus of Fossil HexactinelUd Sponges. 9 



guished into spicule and fibre, but appears homogeneous 

 throughout. But in deciduous s,^Q.c\mQns oi AjjhrocalUstes and 

 Farrea the original spicule undergoes a process of absorption 

 and disappears, leaving in its place a hollow sexradiate cavitj 

 readily observable in the interior of the fibre. Our sexradiate 

 fibre has, then, in the fossil condition a structure essentially 

 identical with that of the recent skeleton of Farrea when in a 

 deciduous state. The siliceous fibre of our fossil corresponds 

 with the siliceous fibre of Farrea ] and the sexradiate canals 

 in its interior correspond with the hollow casts of the spicules 

 in the latter : the only difference is that the canals in our 

 fossil are continuous from one node to another, while in recent 

 Hexactinelli daj they terminate blindly, as casts of spicules 

 naturally would, their blind terminations generally overlap- 

 ping one another *. But even this difference vanishes with 

 a close examination of the fossil fibre, as I shall show when 

 we come to speak of the various modes of its fossilization. 



The characters of the sponge already described are sufficient 

 to define the genus, which I now propose to call " Stauronema,^^ 

 from the cross-like disposition of the thick skeletal fibres about 

 the nodes of the network, a feature readily visible under a 

 common hand-lens. In the oscular plate the nodes of the 

 network are usually arranged symmetrically at equal distances, 

 from each other, so as to form meshes which would be cubical 

 but for the thickening of the fibre towards the node, which 

 converts the cubes into spheres or ellipsoids. By reason of 

 the symmetrical grouping of the nodes, the skeletal fibres fall 

 into three series : — one longitudinal, ascending from the base ; a 

 second horizontal, radiating from the imaginary axis on which 

 the half-cone of the sponge may be supposed to be described ; 

 and a third horizontal and concentric with the curve of the 

 fan. 



The longitudinal fibres (PL II. fig. 4, I) deviate from a 

 parallel course by diverging, as they rise from the base, 

 towards the anterior and posterior faces of the plate ; and to 

 maintain the uniform size of the meshes, fresh sexradiate 

 elements are interposed in the same way as I have described 

 in Euhroclius and the Ve ntriculites f. The radiating fibres, 

 since the curve of the fan is gentle and the oscular plate thin, 

 lie in almost parallel lines ; but both they and the concentric 



• [As the absorption goes on, the form of the spicules becomes lost," 

 and that which remains is a simple cylindrical cavity, which led Bower- 

 bank to say that the fibre of Farrea was channelled like that of the Cera- 

 tina, ex. gr. Lufaria. — Note hy Mr. Caetek.] 



t Quart. Journ. Geol. Soc, Feb. 1873, p. m, fig. 4 ; Geol. Mag., Sept. 

 1876. 



