SIDONOPS AXGULATA. 35 



5.5-6 n long and 4 fi thick. In the ordinary strongylosphaerasters with from 

 ten to twenty equal rays, which occur in great numbers in all the specimens, 

 the rays are 2-4 /< thick, in var. microana 3-9 /(, and in the other two varieties 

 1-7 /i long. 



The size of the centrum and of the whole aster is, like the size of the rays, 

 in inverse proportion to the number of the number of the latter. In the strongy- 

 losphaerasters of var. megana and var. orthotriaena with from one to nine fully 

 developed rays, the centrum is 11-13 fx and the whole aster 21-26 fi in diameter. 

 In the strongylosphaerasters with from ten to twenty equal rays these dimensions 

 are 7-14 /( and 14-24.5 n respectively in all the three varieties. 



The distal parts of the fully developed rays bear numerous spines while 

 their basal part and the centrum are smooth-. The spines are conic, usually 

 0.5-1 /( long, rarely smaller, and not recurved. Those situated on the terminal 

 face of the ray appear to radiate from a common centre within the tip of the ray 

 and diverge accordingly; those on the sides of the ray are slightly oblique, 

 inclined towards the end of the ray. When the ray is reduced in length the 

 spines on its terminal face retain their full size. Consequently the low protuber- 

 ances representing greatly reduced, rudimentary rays of this kind, are covered 

 with tufts of spines (Plate 14, figs. 18, 19, 29, 30). 



In the rays of some of the strongylosphaerasters of var. orthotriaena I ob- 

 served thick axial threads. These were joined in a regularly concentric manner 

 in the centre of the spicule and extended in straight lines along the axes of the 

 rays to within a short distance of their ends, where they appeared to terminate 

 with slight irregular thickenings. Occasionally it seemed that exceedingly 

 fine branches extending towards the spines arose from the distal parts of these 

 axial threads. These being near the limit of microscopic visibility, it is doubt- 

 ful whether such structures really exist, or whether the impression of them was 

 merely an optical illusion. 



Besides the strongylosphaerasters described above I found in the centrifugal 

 spicule-preparations of the massive specimen of var. megana some small ones, 

 7.5-14 /( in total diameter with a centrum measuring only 1.5-2.5 /«, and seven- 

 teen to nineteen minutely spined rays 0.5-1 n tliick. These asters appear to be 

 young stages of the ordinary strongylosphaerasters. 



The sterrasters (Plate 13, figs. 22-25; Plate 15, figs. 1-3, 5-12) are more or 

 less regular flattened ellipsoids. When seen from above, with the umbilicus 

 in the centre of the upper side, their contour generally appears as a regular 

 ellipse, sometimes nearly approaching a circle (Plate 15, figs. 9-11). In var. 



