SIDONOPS OXYASTRA. 



45 



choanosomal few-rayed forms, without large centrum (large oxyasters); large 

 subcortical many-rayed forms, with large centrum (large oxysphaerasters) ; and 

 small, dermal, mostly many-rayed forms, with or without large centrum (small 

 oxyasters and oxysphaerasters). 



The large choanosomal oxyasters (Plate 7, figs. 3-5a, 7, 8, 13-15) have a 

 slight central thickening 2.5-4.5 n, that is two to three times the basal thickness 

 of the rays, in diameter, and from four to ten, most frequently seven, straight, 

 conical and pointed or blunt, concentric, and quite regularly distributed rays. 

 With the exception of its proximal end, the whole of the ray is covered with 

 rather large and uniformly distributed spines (Plate 7, figs. 13-15). The rays 

 are 11-25 /< long and at the base usually 1.2-2 /< thick, the total diameter of 

 the aster being 18 45 /«. A few asters of this kind, with much thinner rays also 

 occur. These asters, which are less than 20 /< in total diameter, have rays, at 

 the base, only 0.5-0.7 p. thick. They are probably young forms. 



The large subcortical oxysphaei^asters (Plate 7, figs. 3c, 19, 20) have a spherical 

 centrum 4.2-6.5 fi, that is from a quarter to a third of the whole aster, in diameter, 

 and from sixteen to twenty-three concentric and regularly distributed rays. 

 The rays are straight, conical, sharp pointed, covered with rather large spines, 

 6-7 p. long and at the base 1.1-1.4 fi thick. The whole aster is 16-22 fi in 

 diameter. 



The small dermal oxyasters and oxysphaerasters (Plate 7, figs. 3~5b, 9-12, 

 16-18) form a continuous series. One end of this series is represented by forms 

 which have hardly any central thickening at all and appear as true oxyasters 

 (Plate 7, figs. 9 and 10 right above, 18). The other end of the series is represented 

 by forms with a centrum more than a third of the whole aster in diameter. The 

 small dermal oxyasters and oxysphaerasters have from nine to eighteen straight, 

 conical, and regularly distributed rays. The rays always appear to bear nu- 

 merous small spines. Often however these spines are so minute that they can- 

 not be made out as such, a roughness of the ray then being the only indication 

 of their presence. The rays are (without the centrum) 2-4.5 n long and at the 

 base 0.7-1.5 /« thick, the total diameter of the aster being 6-13.5 /<. 



The sterrasters (Plate 6, fig. 21; Plate 8, figs. 1-3, 6-12) are flattened ellip- 

 soids, 76-85 /( long, 66-73 p broad, and 50-64 ,u thick, the average proportion of 

 length to breadth to thickness being 100 : 90: 76. 



Very young sterrasters, some hardly 10 /( in diameter, were observed. 

 These appear as spheres composed of equal and regularly distributed, im- 

 measurably thin, straight, radial rays. In a spicule-preparation I found a 



