Genus of Fossil HexactinelUd Sponges. 7 



to which I have chiefly trusted, only using the former when 

 the hitter has not been avaiUible. The sections I have iiad 

 made have been taken along the following planes : — (1) longi- 

 tudinal and at right angles to the surface, both through the 

 centre and nearer the sides — longitudinal sections (1*1. II. 

 fig. 2) ; (2) transverse and at right angles to the surface — 

 transverse sections (PI. II. tig. 1) ; (3) parallel to the sur- 

 face, one through the oscular plate and another through the 

 posterior mass — parallel sections (PI. II. fig. 1,6, c, fig. 3). 



The aj)pcarances of these sections under the microscope I 

 shall now describe, and in so doing shall confine myself first 

 to an account of the skeletal structure which they demonstrate, 

 referrinir most of the facts which bear on the mineral charac- 

 ters to a subsequent paragrapii. 



Each of the sections we have defined shows a regular net- 

 work of fibres arranged in the following manner. Selecting 

 a single node in the net we observe four fibres, usually sili- 

 ceous, radiating from it at right angles to one another in the 

 form of a cross (figs. 1, 2, 3) ; each is perfectly continuous 



Fi?l. 



Fie:. 2. 



§10 



ftc^ 



Sections taken throup^h the oscular plate of Stauronema Carteri, from the 

 specimen represented in transverse section on Plate II. fig. 1 ; all 

 magnified 30 diameters. Fig. 1. Longitudinal section {a, PI. II. 

 fig. I). Fig. 2. Transverse section (PI. II. fig. 1). Fig. 3. ParaUel 

 section (c, PI. 11. fig. 1). 



with similar fibre from an adjacent node, and has at its 

 greatest distance from the two nodes it connects (^'. e. at a 

 point midway between the two) a diameter of -pi-o to -pg-ix o*' 

 an inch ; but on approaching the node it thickens considerably 

 BO as to fill up the angles of the cross and round them off : in 

 this way the meshes of the net, which, from the disposition of 

 the nodes, would otherwise be rectangular, are always round 

 or oval ; and these rounded spaces, which are bounded by the 

 outer margins of the fibres, are so sharply defined as to enable 

 us to state w^ith certainty that the fibres themselves are per- 

 fectly smooth and not in any way spined. 



