20 C. F. JENKIN. 



Fig. 57. The gastral skeleton consists of quadriradiates with the basal ray pointing 

 downwards : the body skeleton consists of sub-gastral triradiates and the inner 

 ends of the projecting oxea. Among the sub-gastral triradiates there are a very few 

 quadriradiates exactly like the triradiates, except for the development of the apical ray, 

 which is roughly perpendicular to the plane of the other rays, and does not project into 

 the gastral cavity. 



The dermal skeleton consists of a thin layer of triradiates with the basal ray 

 pointing downwards. 



The space between gastral and dermal membranes is filled up with three or 

 four layers of spherical flagellated chambers between which are small incurrent 

 and excurrent canals. There are no large incurrent chambers as in Leucandra 

 cirrata. 



The oscular skeleton consists of quadriradiates with an oral angle of nearly 180° and 

 oxea. The dermal triradiates do not reach quite up to the edge. The thin oxea which 

 form the fringe are straight, with hastate ends. Just below the oscule there are a few 

 very short stout projecting oxea, similar in form to the longer ones lower down. 



Spicules (Fig. 58). 

 The Oxea are of one sort : — 



(rt) Large projecting oxea, bluntly pointed at the inner end, hastate and 

 sharply pointed at the outer end, nearly straight for about three-quarters 

 of their length, then bent at the outer end, 260 /« to 670 m long x 12 m to 

 24 m thick at the thickest point. The thickness varies irregularly along 

 the length. 



Triradiates. — There are two sorts of triradiates :— 



{h) Sub-gastral alate triradiates. Basal ray straight, tapering uniformly to a 

 sharp point, 160m to 200m long x 6m thick. Paired rays equal, more or 

 less bent downwards, 80m long x 6m to 8-m thick, nearly uniform in 

 thickness all along, then bluntly pointed. Oral angle 135° to 150°. 



(c) Dermal alate triradiates. Basal ray straight, occasionally shorter than the 

 paired rays, 60m to 200m long x 8m to 11m thick, tapering slightly, then 

 bluntly pointed. Paired rays usually differing slightly in shape and size ; 

 bent upwards in a wide sweep at the centre and then usually slightly 

 downwards near the points. 100 m to 140 m long x 9 m to 12 /x, thick. 

 Oral angle 105° to 115°. 



The Quadriradiates are of one sort : — 



(fZ) Gastral alate quadriradiates. Basal ray straight, tapering uniformly to a 

 sharp point. 100 ^ to 300 /a long x 8 /n to 12 ju thick. Paired rays equal, 

 bending upwards in a wide sweep at the centre, outer half straight, 80 /a to 



