EURETE SPINOSUM. 137 



thickened by the apposition of silica-layer, together with the beams of the skele- 

 ton-net which had, as it were, incorporated them. 



The rhabds, which may, as above mentioned, be foreign, are long, smooth 

 centrotyles. They are about 15 yu thick near the centre. The tyle measures 

 about 17 ju in transverse diameter. 



The superficial ■pentactines (Plate 29, figs. 20-22) usually have fairly equal, 

 straight, conic, terminally rounded lateral rays, which enclose angles of 90° with 

 their neighbours (Plate 29, fig. 20). Rarely (Plate 29, fig. 21) the lateral rays 

 are cyUndroconic, curved, and irregularly arranged. The lateral rays are 150- 

 270 M long, 12-22 /i thick at the base, and covered throughout with vertically 

 arising spines. The spines on the proximal part of these rays are 8-12 ii long; 

 distally they become smaller. The apical (proximal) ray is smaller than the 

 lateral rays and destitute of large spines. 



The hemioxyhexasters (Plate 29, figs. 9-17, 26b) measure 80-122 n in total 

 diameter, usually 90-110 m. Two of their rays, which extend in the same axis 

 and lie opposite each other, are usually conic, short and simple, and only excep- 

 tionally bear an end-ray. These two rays I designate the apical. The four 

 other rays, which lie in a plane vertical to the axis of the other two, nearly always 

 bear end-rays. These four rays I designate the lateral. The simple stems 

 (main-rays) of these lateral rays always enclose angles of 90° with their neigh- 

 bours and are, in the same spicule, usually fairly equal (Plate 29, fig. 10) ; only 

 exceptionally they differ in length (Plate 29, fig. 12). In the ordinary regular 

 hemioxyhexasters the lateral main-rays are 16-24 fi long; in the rare irregular 

 forms the shortest is sometimes only 14 fx long, or still shorter. The lateral 

 main-rays are cyUndroconic, at the base 3.5-6.5 m thick, usually about 4 fi, and 

 uniformly attenuated towards the end, the transverse diameter of which is about 

 three quarters of that of the base. The thickness of these main-rays is not in 

 proportion to their length, the thickest not being longer, often indeed shorter 

 than the thinner ones. The lateral main-rays bear minute spines which decrease 

 in size proximally. In the thinner ones these spines can only be made out in the 

 distal part, and also here only in the u. v. photographs (Plate 29, fig. 9). The 

 thick ones are covered with clearly visible spines throughout (Plate 29, fig. 12). 



Each lateral main-ray bears three regularly disposed end-rays which lie in 

 the plane of the four lateral main-rays. One of them extends in the same direc- 

 tion as the main-ray to which it belongs, and appears as a continuation of the 

 latter. The other two lie symmetrically on the two sides of this central one. 

 These regularly disposed end-rays are, in the same spicule, usually equal (Plate 



