118 STAUROCALYPTUS HAMATUS. 



The discodasters (Plate 16, fig. 39e; Plate 17, figs. 9d, lOd, 11, 12, 13d, 14- 

 25) measure 58-320 fi in diameter, usually 70-260 m- 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 m 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 

 sUghtly curved, convex to the continuation of the pseudomain-ray axis, and 

 diverge from it at angles of 12-16°. They are 16-1 15 ^ long, 0.7-2 ^ thick at the 

 base, and attenuated towards the end, where they measure 0.4-1.5 m in transverse 

 diameter. They bear, along their length, very obliquely situated, backwardly 

 directed spines, which are sometimes 1.5 m 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 M 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 ^ in the 

 largest, and to a small extent also to differences in the length of the pseudomain- 

 rays, 16 M in the smallest, 49 m in the largest. The main-rays are in the largest 

 discoctasters only 3 ^ 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 fx thick at the base. They are attenuated distally and 



