32 



SIDONOPS ANGULATA. 



The various oxyasters and oxysphaerasters (Plate 14, figs. l-15a, b, c, 16, 

 23, 24) form a continuous series. They have from one to twenty-three rays. 

 Forms with more than thirteen rays are much more frequent than forms with 

 fewer, and among the latter those with from one to three rays much scarcer than 

 those with four or more. These asters form a fairly continuous series ranging 

 from large oxyasters without centrum and few rays (Plate 14, fig. 24) to small 

 oxysphaerasters with large centrum and numerous rays (Plate 14, figs. 16, 23). 

 The rays are straight, usually on the whole conical, attenuated towards the sharp- 

 pointed end in the basal and middle parts more gradually than in the terminal 

 part, and perfectly smooth (Plate 14, figs. 16, 23, 24). Their shape varies in 

 correlation to their size, the longest rays being the most slender and the shortest 

 the stoutest. Sometimes, chiefly in the two- to four-rayed oxyasters (Plate 14, 

 figs. 3b, 13b), short, rounded or truncate rudiments of reduced rays occur in 

 addition to the properly developed conical and pointed ones. These ray- 

 rudiments are generally smooth, rounded knobs not longer than broad (Plate 

 14, fig. 3b), more rarely longer and in this case sometimes crowned with a few 

 terminal spines. The rays are always concentric. Their arrangement is in 

 the few-rayed oxyasters often irregular, in the many-rayed ones nearly always 

 regular. In none of the two-rayed forms observed were the two rays regu- 

 larly arranged, that is, situated exactly opposite each other in a straight line, 

 the angle enclosed by them being always considerably less than 180°. In 

 several of these asters this angle was under 120° and in one even under 90°. 

 These diactine asters consequently look like more or less opened compasses. 

 Also in the three-rayed oxyasters (Plate 14, figs. 3b, 13b, 24) irregular ray- 

 arrangement is the rule. In the oxyasters with four (Plate 14, fig. lb) or more 

 (Plate 14, figs. 4b, 16, 23) properly developed pointed rays, on the other hand, 

 the rays are usually regularly arranged. 



The oxyasters and oxysphaerasters are 11-64 fi in total diameter, the cen- 

 trum attaining a maximum diameter of 12 fi. The normal conically pointed 

 rays are 2.5-40 /< long, and at the base 1.6-5 fi thick. Roughly speaking, the 

 size of the rays and of the whole aster is in inverse proportion to the number 

 of rays. Only the monactine oxyasters appear as an exception to this rule. 

 Since however these spicules are rare and I was able to measure but few, I 

 do not attach much importance to this fact. The one- to five-rayed oxyasters 

 are 25-64 /^ in diameter, their normal conical rays measuring 13-40 by 1.7-5 /x. 

 The six- to ten-rayed oxyasters are 23-44 // in diameter, their rays measuring 

 8.5-25 by 1.6-4.5 n. The eleven- to twenty-three-rayed oxyasters are 11-25 fj. 

 in diameter, their rays measuring 2.5-13 by 1.7-3.3 fi. 



