SIDONOPS CALIFORNICA. 



21 



consider them as derivates of ordinary mesoproclades produced by a change in 

 the position of the clades. 



The large choanosomal oxy asters (Plate 5, figs. 21 -26c, 31c, 28, 29) have 

 from six to fourteen, rarely as many as twenty straight, conic, and quite uni- 

 formly distributed rays. There is no central thickening. The rays vary from 

 sharp pointed (Plate 6, fig. 28) to blunt (Plate 5, fig. 29). Apart from the very 

 short, smooth, basal parts, the rays are entirely covered with spines, which 

 appear to be directed backwards, towards the centre of the spicule. On the 

 apex of the blunt rays a terminal spine is usually observed (Plate 5, fig. 29). 

 The rays are at the base 1.7-3 n thick. The whole aster is 22-48 fi in diameter. 

 An inverse proportion between size and number of rays is clearly pronounced. 

 The oxyasters with six to eleven rays are 27-48 fi, those with twelve or more 

 rays 22-26 n in diameter. 



The rare, small oxysphaerasters (Plate 6, fig. 30b) have stout, pointed, 

 conical, spined rays and measure 7-9 in total diameter. 



The small strongylosphaerasters (Plate 5, figs. 24a, 26a, 30a, 31a, 32-35) 

 have a centrum 2-3.5 ^ in diameter, from which six to seventeen rays arise. 

 These are usually regularly, more rarely irregularly distributed and 1.6-2.8 n 

 long. They are cylindrical, 0.8-1.5 /« thick, and rounded at the ends. They 

 always bear spines, which are either quite uniformly distributed or massed at 

 the ends, where a verticil of larger spines sometimes appears to be present. 

 A conspicuous terminal spine, arising from the end of the rays, is often ob- 

 served. The whole aster measures 4.5-9 /< in diameter. A correlation between 

 number of the rays and size is not discernible. 



The sterrasters (Plate 5, figs. 20, 36, 37) are flattened eUipsoids, 116-130 n 

 long, 97-105 n broad, and 70-90 /< thick. The proportion of length to breadth 

 to thickness is usually about 100 : 81 : 69. In the centre a nearly spherical 

 cluster, 4 // in diameter, composed of numerous small but conspicuous granules, 

 is met with. In the youngest sterrasters observed, which appear as spheres of 

 slender rays, 18 n in diameter, this central cluster of granules has the same 

 size and structure as in those fully developed. The umbilicus lies in the centre 

 of one of the flat faces of the ellipsoid. It is about 12 ^ deep and 12 to 15 /j. 

 broad. In the great majority of sterrasters the free distal ends of the rays are 

 uniformly distributed, 2-3 n thick and 1.5-2 /x apart. In a small minority, 

 perhaps 2 % of all the sterrasters, these free ray-ends are irregularly distributed 

 and in some places much farther apart, more or less extensive parts of the sur- 

 face of such sterrasters being free from them. These altogether rayless parts of 



