8 Mr. W. J. Sollas on Stauronema, a npw 



In the centre of the node is a small and very definite circle, 

 ^^ to -5^-i-T5 inch in diameter (figs. 1, 2, 3, c), which is produced 

 by the section crossing at right angles a cylindrical tube, 

 originally hollow, but now generally tilled with carbonate of 

 lime ; and from this radiate four similar cylindrical canals, one 

 in the axis of each arm of the cross ; these, of course, are seen 

 sideways and not end on, and ordinarily they arc continuous 

 from one node to another, like the tibre in which they are 

 excavated. As these appearances are to be seen equally in 

 each of three sections taken at right angles to each other 

 (figs. 1, 2, 3), it is,clear that our quadrilateral cross of fibre is 

 really a sexradiate one (fig. 4), with its arms arranged about three 



Fier. 4. 



-6 



Diagram of the network of Statironema. Scale 60 : 1. a, sexradiate 

 canal ; h, sexradiate fibre. 



axes at right angles to each other, and that corresponding 

 with the axes interiorly is a similar sexradiate hollow canal. 



Kow this structure is exactly that which characterizes the , 

 rete of the Vitreohexactinellida, and may be seen to perfection, 

 with differences merely as to detail, in deciduous skeletons of 

 Farrea and Aphrocallistes. In these genera, as in the Vitreo- 

 hexactinellidee generally, the skeleton is produced by a growth 

 of siliceous matter over sexradiate spicules ; and in Farrea occa 

 each node of the resulting network is a rectangular sexradiate 

 cross of fibre, which has formed about a sexradiate spicule, 

 which thus comes to occupy the centre of the fibre. In many 

 vitreous hexactinellids the fundamental spicule is preserved 

 imbedded in the siHceous fibre, which is thus originally solid ; 

 and which, as it is composed of the same material all through, 

 without any difference of refractive index, cannot be distin- 



