STAUROCALYPTUS HAMATUS. 117 



or unequal. The rays are, measured along the chord, 75-120 ix long, and 2.5- 

 8 ix thick at the base. They are conic and gradually attenuated to a fine point. 

 The proximal parts of the rays are straight and regularly arranged so as to enclose 

 angles of 90° with their neighbours. At a distance from the centre usually equal 

 to from one half to three quarters of the length of the chord of the whole ray, 

 the rays begin to curve either gradually or more often abruptly with a distinct 

 angular bend. The distal part of the ray, beyond this point, is uniformly 

 curved through an angle of at least 90°, usually more. Sometimes the curvature 

 is so great that the end points directly backwards and the end-tangent becomes 

 nearly parallel to the axis of the basal part of the ray (Plate 17, fig. 1). Excep- 

 tionally the curved end-part forms nearly a whole turn (Plate 17, fig. 10c, the 

 upper ray). In such cases it is clearly to be seen that the curvature is spiral, 

 and it seems probal:)le that it is of this nature also in those cases where the curved 

 part of the ray is shorter, and the true nature of its curvature not so clearly 

 discernible. 



Like the simple rays of the straight-rayed hemioxyhexasters and oxyhexac- 

 tines described above, the rays of these spicules are smooth at the base, and 

 farther on covered with slender, oblique, backwardly directed spines, which 

 decrease in size distally, so that the end-part appears merely roughened or nearly 

 smooth. 



Of hemioxyhexasters with hook-like rays I found only two or three. These 

 had one bifurcate and five simple rays. One of these spicules measured 210 m 

 in diameter; its simple rays were 3 n thick at the base. 



The small discohexasters (Plate 17, fig. lOe; Plate 18, figs. 1-4, 7, lib, 12b) 

 measure 20-23 n in total diameter. The main-rays of the same sjncule are equal 

 and enclose angles of 90° with their neighbours. A central thickening, 3-4 jx in 

 diameter, can clearly be made out. The main-rays are smooth, 3.5-4.5 n long, 

 1.2-1.6 M thick in the middle, and thickened at both ends, proximally to the 

 centrum, distally to the somewhat extended base, from which the end-rays arise. 

 Each main-ray bears about 16 end-rays. The end-rays are curved, concave to 

 the continuation of the main-ray axis, quite considerably at the base, but only 

 very slightly, or not at all, towards the end. They are 7-8 m long, about 0.2 ^ 

 thick at the base, and attenuated towards the end, which bears a thickening 

 about 0.8 n in transverse diameter. This terminal thickening is certainly 

 broader than high and convex on the outer side. However, in consequence of 

 its small size more cannot be made out about its shape. This thickening may 

 be, and, judging by analogy, probably is, a verticil of terminal, recurved spines. 



