HOLASCUS EDWARDSII. 
27 
together are slender-rayecl hexactines. The most frequent forms are triactines 
with a central thickening. Usually the rays are either well-developed and 
very long, or reduced to mere knobs arising from the centre. Rays intermediate 
between these extremes (Plate 18 , fig. 23) are rare. The rays have a maximum 
length of 7 mm., are very slender, only 6-14 /x thick, usually nearly cylindrical 
and terminally rounded, more rarely considerably attenuated towards the end. 
The central thickening, which is composed of the knob-like rudiments of the 
aborted rays, is 15-32 /x in diameter. 
The small choanosomal hexactine megascleres have straight, conic, pointed 
rays, 0.17-1 mm. long and 12-18 /x thick at the base. 
The hypodermal hexactines (Plate 18 , figs. 19-21, 24-26) have a more or less 
curved proximal ray, usually 1.2-1. 5 mm. long and 5-7 /x thick at the base. 
The lateral rays are fairly straight, have the same basal thickness, and are 
usually 180-240 /x long. The distal ray is 180-260 /x long, straight, and 6-13 m 
thick at the base. It is more or less club-shaped, thickened above, and abruptly 
pointed. At the thickest point, which is usually about 50 /x from the end, the 
distal ray measures 13-23 n, on an average (of twelve measurements) 18 n in 
transverse diameter. The proportion of the basal to the maximum thickness 
is 1 : 1 to 1 : 3, usually about 1 : 2. The distal ray is covered with spines. These 
are small and scarce below but become larger and more numerous above, towards 
the distal end. The spines are broad, conic, and pointed, with a maximum length 
of 2.5 fx and are directed obliquely upward, towards the end of the ray. On 
account of their relatively great breadth and their obliquity, they appear as 
nose-like protuberances of the ray. 
The hypogastral pentactines (Plate 18 , figs. 17, 18) are similar to the hypo- 
dermal ones but their distal rays are distally much less thickened. The 
maximum thickness of their distal rays is only 7-20 fx, on an average (of twelve 
measurements) 13 fx. The proportion of the basal to the maximum thickness 
is 1 : 1 to 1:2. 
The abundant oxyhexasters, the rare hemioxyhexasters, and the still rarer 
microoxyhexasters (Plate 19 , figs. 1 , 2) are obviously all different forms of the 
same kind of spicule. They measure 108-180 /x in total diameter. A difference 
in the size of forms with simple and with branched rays could not be detected. 
The main-rays, which are, in the same spicule, equal, and enclose right angles 
with their neighbours, are 8-12 n long and 2-4 /x thick. Each one bears from 
one to four end-rays. The number of end-rays on the six main-rays of the same 
spicule is usually unequal; but the difference is generally only one, main-ray 
