HOLASCUS EDWARDSII. 27 



together are slender-rayed 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 /^ 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 ^ in diameter. 



The small choanosomal hexadine megascleres have straight, conic, pointed 

 rays, 0.17-1 mm. long and 12-18 fx 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 ^ thick at the base. 

 The lateral rays ate fairly straight, have the same basal thickness, and are 

 usually 180-240 ^ long. The distal ray is 180-260 tx long, straight, and 6-13 ^ 

 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 m, 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 iJL 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 ix, on an average (of twelve 

 measurements) 13 m- 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 ^ 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 fx long and 2-4 ^ 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 



