162 HYALONEMA (HYALONEMA) OBTUSUM. 



short rays relatively much stouter ones than among the long rays. The rays 

 most strongly reduced, that is those under 55 ai in length, are by far the relatively 

 stoutest. None of the rays as short as this was in gracilis under 25 ix, several of 

 them were here 35 n thick. 



In both varieties the tips of the rays bear broad, conic, vertical spines with a 

 maximum length of 4 /i (Plate 36, figs. 27, 28). On the distal thickening these 

 spines are usually densely crowded (Plate 36, figs. 6, 7, 9), more rarely sparsely 

 scattered (Plate 36, fig. 4, fig. 18, the upper and lower ray, fig. 23, the upper and 

 left ray). They are usually confined to the distal thickening, the proximal part 

 of the ray and the conic tip (when present) protruding beyond it being quite 

 smooth (Plate 36, figs. 1, 2, 4-25, 27, 28). Sometimes, however, the spines cover 

 the whole ray (Plate 36, fig. 3) in greater or smaller numbers. 



It is to be noted that the axial thread is in many of these tetractines, particu- 

 larly in the slender-rayed (perhaps young) ones, remarkably wide (Plate 36, 

 fig. 27) , sometimes as much as half as thick as the ray itself. 



The triactines (Plate 39, fig. 21) are obviously tetractine-derivates, in which 

 one of the rays has quite disappeared. They are more frequent in var. robusta 

 than in var. gracilis, measure in both 330-760 ju in total length, and have rays 

 about 37-42 n thick in the former, and about 20 /x tliick in the latter. 



The diactine and monactine rhabds are of two kinds: — shorter and stouter 

 tetractine-derivates, and longer and more slender derivates of the ordinary 

 rhabds of the choanosome. 



The tetractine-derivate rhabds of the cement-mantles (Plate 36, figs. 31, 

 40-45; Plate 39, figs. 17, 34-38) are generally diactine and sUghtly and irregu- 

 larly curved, rarely (Plate 39, fig. 17) strongly angularly bent. Such strongly 

 bent, compass-like spicules have only been found in var. robusta. In var. 

 gracilis some small spicules, also apparently belonging to this group, have been 

 observed, in which the curvature is so great that one half of the spicule forms a 

 complete circle (Plate 36, fig. 45). The tetractine-derivate rhabds in var. 

 gracilis are 170-950 m long and 11-39 n thick near the middle; in var. robusta 

 450 ,u-1.42 mm. long and 6-50 ti thick near the middle. In the shorter spicules 

 of this kind a central tyle is usually present, but the longer ones are often without 

 it (Plate 39, figs. 34, 35, 37). When present the central tyle is, in var. gracilis, 

 as much as 10 m, and, in var. robusta, as much as 20 fi, thicker than the adjacent 

 parts of the spicule; in transverse diameter they measure in the former 13-39 fi, 

 and in the latter 25-70 ii. It either passes gradually into the body of the spicule 

 (Plate 36, figs. 40, 44; Plate 39, figs. 36, 38) or it is set off from it more or less 

 distinctly (Plate 36, figs. 31, 41-43). Most of these spicules are fairly isoactine, 



