298 HYALONEMA (PRIONEMA) FIMBRIATUM. 



with the true pinules. The slender-rayed long-spined basals may therefore be 

 considered as pinule-derivates. 



The acanthophores with stout rays are monactine to pentactine. 



Among the tri- to pentactine acanthophores (Plate 63, figs. 15-19) the tetrac- 

 tines with four rays extending in one plane (stauractines) greatly predominate. 

 The maximum diameter of these spicules usually is 330-670 m, but much smaller 

 forms only 100-300 ^ in diameter (Plate 63, fig. 15) also occur. The rays are 

 equal or unequal, fairly straight, 10-28 m thick at the base, and slightly attenu- 

 ated distally. They often have a somewhat wavy outline. Their end-parts 

 are usually thickened, densely covered with rather short, broad spines, and 

 blunt-pointed, or, particularly in the cases where the rays are reduced in length, 

 terminally rounded. The thickened, spiny end-parts of properly developed 

 long rays are accordingly usually spindle-shaped, those of reduced, short rays 

 usually more or less spherical. 



Of the diactine or monactine forms, that is the rhabd acanthophores, various 

 kinds can be distinguished : — slender, long-spined tylostyles ; angularly bent 

 centrotyle diactines; straight or slightly curved, stout, centrotyle diactines; 

 stout, strongly curved, not centrotyle rhabds with thickened ends; slender, 

 slightly curved, long rhabds with thickened ends; and strongly curved rhabds 

 of the last mentioned sort. 



The slender tylostyle monactine acanthophores with long spines are very rare. 

 A spicule of this kind measured was 220 fx long and 8 m thick ; its terminal tyle 

 measured 18 n in diameter. I am inclined to consider these spicules as monac- 

 tine-derivates of the slender-rayed long-spined basal tri- to hexactines above 

 described. 



The angularly bent centrotyle diactine acanthophores are also very rare. They 

 have a spiny tyle and two straight rays enclosing an angle of about 90°. In one 

 of these spicules measured, the two actines were quite straight, and respectively 

 13 fi thick and 430 and 500 m long. The spiny central tyle was 28 m in diameter. 



The stout, straight or slightly curved, centrotyle diactine acanthophores (Plate 

 63, figs. 20-23) are usually 550-920 m long, and 14-30 yu thick near the centre. 

 The central tyle is 20-54 /^ in diameter. The proportion of the thickness of the 

 tyle to the thickness of the adjacent parts of the spicule is 114-300 : 100. When 

 the tyle is large, it is spiny (Plate 63, fig. 20) ; when it is small, it is smooth 

 (Plate 63, figs. 21-23). The two rays of these spicules have the same shape as 

 the rays of the tri- to pentactine stout-rayed basals above described, but are 

 on the whole longer. I think there can be no doubt about these spicules being 

 diactine-derivates of the stout-rayed tri- to pentactine acanthophores. 



