SIDONOPS AXGULATA. 33 



The development (size) of the centrum is, roughly speaking, in true propor- 

 tion to the -number of rays and in inverse proportion to the size of the aster. 

 In some of the large two- to five-rayed oxyasters the knob-like ray-rudiments 

 clustering round the centre form an irregular thickened mass, but none of these 

 asters have a true centrum. The largest oxyaster with such a centmm observed 

 was 38 n in diameter. Most of the six- and seven-rayed oxyasters are also 

 without centrum, but among the oxyasters with eight to ten rays a great many 

 are provided with one, and in the oxyasters with eleven or more rays a spherical 

 central thickening is invariably present. In the larger oxyasters with centrum 

 the diameter of the latter is always much less than the ray-length, while in the 

 smallest oxyasters (oxysphaerasters), which also possess the greatest number 

 of rays, the diameter of the centrum considerably exceeds the ray-length. 



The oxyasters and oxysphaerasters of the four specimens are very similar. 

 The dififerences observed, which are recorded in the appended table (page 3-4), 

 are well within the limits of the accidental inaccuracies due to the smallness 

 of the number (only about one hundred) of oxyasters measured. 



The strongylosphaerasters (Plate 14, figs. Id, 2d, 5d, 7d, 9d, lOd, 17-19, 

 25-30) usually have a spherical centrum and about ten to twenty radial rays. 

 In most of the strongylosphaerasters all the rays are about equal in size (Plate 

 14, figs. 17, 25-28). In not a few strongylosphaerastei-s of var. megana and var. 

 orthotriaena however, some of the rays are reduced to insignificant protuberances 

 of the surface of the centrum and are much shorter than the others. On Plate 14 

 two strongylosphaerasters of this kind are represented, one (Figs. 18, 19) with 

 all but three, the other (Figs. 29, 30) with all but one ray thus reduced. The 

 properly developed rays are cylindrical or cylindroconical and truncate, very 

 rarely conical and pointed. In var. microana they are on the whole more 

 slender, longer, and distally more attenuated than in the other two varieties, and 

 the strongylosphaerasters with pointed, conical rays have been observed only 

 in this variety. The rays are 1-12 n long and at the base 2-6 n thick, the 

 dimensions of the fully developed ones being in inverse proportion to their 

 number. Strongylosphaerasters with only one fully developed ray are rare and 

 have been found only in var. orthotriaena. In these the single ray is 12 // long 

 and 6 n thick. In the strongylosphaerasters with two or three fully developed 

 rays, which are quite frequently met with in both specimens of var. megana and 

 in the specimen of var. orthotriaena, these rays are 8 /< long and 4-4.5 /« thick. 

 In the strongylosphaerasters with from four to nine fully developed rays which 

 are still more abundant in var. viegana and var. orthotriaena, these rays are 



