SIDONOPS BICOLOR. 



53 



The stwngylosphaemsters (Plate 10, figs. 6b, ll-13b; Plate 11, figs. 1-5, 

 6-8a, 10) have a spherical centrum 4-14 in diameter, from which generally 

 from nine to thirty rays arise radially. Exceptionally there is only one ray. 

 The rays are concentric, regularly distributed, straight, and at the base 0.7- 

 3.5 // thick. They are cylindrical or cylindroconical, truncate, and 1.5-6 /< long. 

 Their terminal face and the distal parts of their sides are covered with numerous 

 small spines; the proximal parts of the rays and the central thickening are usually 

 quite smooth. The only exception to this is the one-rayed strongylosphaeraster 

 found in a specimen from Station 4531, in which the whole of the ray and also 

 the central thickening are covered with spines. The centrum of this spicule is 

 9 /( in diameter and the single ray 2.5 /i thick at the base and 5 /i long. Two- 

 to eight-rayed strongylosphaerasters were not observed, and the nine- to eleven- 

 rayed forms were found only occasionally in specimens from Stations 2981 and 

 4420. The strongylosphaerasters of the specimens from Stations 4531 and 4551 

 have on the whole more rays than those of the others. The total diameter of 

 the strongylosphaerasters is 9-22 /i. 



DIMENSIONS AND NUMBER OF RAYS OF STRONG YLOSPHAERASTF.RS. 



Stations 



2958 



2981 



3168 



4420 



4531 



4551 



Total diameter /i 



11.5-21 



9-22 



11.5-16 



9-22 



11-15 



9-16 



Diameter of centrum /! 



6.5-12 



4-10 



5 . 5-9 . 5 



6-14 



7-10 



4.5-9 



Rays 



length /( 



2 .5-4.5 



2.5-6 



2-3 . 5 



2-4.5 



1.7-5 



1.5-4 



basal thickness /t 



1.5-2.8 



1.5-3.5 



1 . 5-2 . 5 



1.5-3.5 



1.2-2.5 



0.7-3 



Number of rays /i 



12-21 



9-25 



14-22 



10-19 



1.17-27 



18-30 



The sterrasters (Plate 10, figs. 1-5; Plate 11, figs. 11-14) are usually flat- 

 tened ellipsoids, 130-170 n long, 100 133 /< broad, and 77-97 n thick. A few 

 are somewhat rounded, triangular (Plate 10, fig. 1), not oval, in outline. In the 

 ellipsoidal sterrasters the ratio between the length and breadth is on an average 

 100 : 76. The specimens from Stations 2958 and 4531 have on the whole some- 

 what broader (ratio 100 : 78 and 100 : 82 respectively), those from Station 4551 

 somewhat narrower (ratio 100 : 70) sterrasters. The specimens from Station 

 2981 have slightly larger sterrasters than those from the others. The umbilical 

 pit is usually about 12-15 ft deep and situated in the centre of one of the broad 

 faces of the flattened sterraster. The distal free parts of the rays composing 

 the sterraster are 2.5-4 /x thick and about 2 /j. apart. Those surrounding the 



