HYALONEMA (HYALONEMA) PLACUNA. 
209 
large tyle ; a few are diactine centrotyle styles and tylostyles. Hypodermal and 
hypogastral pentactines occur below the outer and inner surface, and hexactine 
megascleres in the interior. Numerous microhexactines and a few diactine 
microhexactine-derivates are found in all parts of the sponge. Seven forms of 
amphidiscs can be distinguished : — 1 , large macramphidiscs with apically broad 
anchors; 2, small macramphidiscs with apically narrow anchors; 3, large, 4, 
medium, and 5, small mesamphidiscs; 6, large micramphidiscs with narrow 
anchors; and 7, small micramphidiscs with broad anchors. In form B all the 
seven kinds of amphidiscs occur. In form A No. 1 (the large macramphidiscs) 
and No. 3 (the large mesamphidiscs) are very rare, and No. 5 (the small mesam- 
phidiscs) are apparently altogether absent. In form B I found a tetraclisc. 
The dermal pinnies (Plate 64 , figs. 8-10, 13a, 14-19; Plate 65 , figs. 22, 23) 
are nearly always pentactine, very rarely hexactine, and have a large, bushy 
distal ray and short lateral rays. Those observed of form A (Plate 64 , figs. 8, 
14-1G) were all pentactine. They have a straight distal ray, 414-475 /x long, 
most frequently about 425-440 fx, and 7.5-10.5 /x thick at the base. This ray 
ends with a stout terminal cone protruding about 20 /j. beyond the tips of the 
uppermost spines. Apart from this terminal cone and the basal end-part., the 
whole of the distal ray is covered with spines. The lowest spines are strongly 
divergent and rather far apart. Distally the spines become more and more 
inclined towards the tip of the ray, and much more crowded. The uppermost 
spines are nearly parallel to the axis of the distal ray. The lowest spines are 
quite short. Distally they increase in dimension, and attain their maximum size 
at from two thirds to three quarters of the length of the distal ray from the 
centre of the spicule. Beyond this point they again become smaller. The 
largest spines are about 40 /x long and 2-3 n thick at the base. It seems that 
the basal parts of these spines are somewhat flattened, their diameter in a direc- 
tion radial to the distal ray being smaller than their diameter in a direction verti- 
cal to this. These distal pinule-rays have the appearance of wheat-ears; this is 
due to a slight, just perceptible curvature of the spines towards the tip of the ray, 
to the increase in their size towards a point in the distal half of the ray, and to 
their density. The maximum thickness of the distal ray, together with the 
spines, is 32-50 m, most frequently 38-47 n. This maximum thickness lies near 
the distal end of the ray. The proportion of the total length of the distal ray to 
the distance between the point of its maximum thickness, together with the 
spines, and the centre of the spicule (the base end of the distal ray) is 100 to 
65.1-83.3, most frequently 100 to 70-78, on an average 100 :73.8. 
