118 
STAUROCALYPTUS HAMATUS. 
The discoctasters (Plate 16, fig. 39e; Plate 17, figs. 9cl, lOd, 11, 12, 13d, 14- 
25) measure 58-320 n in diameter, usually 70-260 /z- They consist of six short 
and stout main-rays, each of which bears several, in the regular forms, four 
end-rays. Eight groups of three of these (24) end-rays, belonging to three 
different main-rays, usually coalesce to as many single rays, which are divided 
distally into verticils of about six terminal branches. The main-rays in the 
same spicule are equal and their axes enclose angles of 90° with those of their 
neighbours. They are distally rounded, 6.5-9 n long and about as thick. The 
six main-rays together appear as a compact central body from which arise six 
dome-shaped protuberances, placed in the positions of the corners of an octa- 
hedron. Seen from above this structure appears, when standing upright (on a 
corner of the octahedron), as a cross with short stout arms (Plate 17, figs. 11, 19- 
23) ; when lying on one of the sides (of the octahedron) it is six-lobed in shape 
(Plate 17, figs. 16-18). The eight coalesced end-ray groups of three arise from 
the eight depressions between the dome-shaped tips of the main-rays, at points 
corresponding to the eight faces of the octahedron. These coalesced end-ray 
groups, which might be designated as pseudomain-rays, are 16-49 /z long and 
3-10 M thick. They are on the whole, cylindrical, but usually somewhat irregu- 
lar, thickened here and there (Plate 17, figs. 14-16). The terminal branches 
of these pseudomain-rays, which may be designated as secondary end-rays, are 
slightly curved, convex to the continuation of the pseudomain-ray axis, and 
diverge from it at angles of 12-16°. They are 15-115 n long, 0.7-2 /z thick at the 
base, and attenuated towards the end, where they measure 0.4-1. 5 ^ in transverse 
diameter. They bear, along their length, very obliquely situated, backwardly 
directed spines, which are sometimes 1.5 /z long and somewhat curved. Their 
end is crowned by a terminal verticil of similar but stouter and more divergent 
recurved spines, which together form a sort of terminal disc with serrated mar- 
gin, 1-2.5 /z in transverse diameter (Plate 17, fig. 24). 
The great differences in the size of the discoctasters is due chiefly to differ- 
ences in the length of the secondary end-rays, 15 n in the smallest, 115 m in the 
largest, and to a small extent also to differences in the length of the pseudomain- 
rays, 16 m in the smallest, 49 /z in the largest. The main-rays are in the largest 
discoctasters only 3 m longer than in the smallest. 
Not infrequently (Plate 17, figs. 13d, 14) a simple ray (end-ray), curved at 
the base and straight farther on, arises directly from the central mass composed 
of the main-rays, between the pseudomain-rays. These simple main-rays are 
27-31 m long, and 1.5-2 /z thick at the base. They are attenuated distally and 
