204 HYALONEMA (HYALONEMA) POLYCAULUM. 



smooth in their basal and spined in their distal part. Those of the large forms 

 are often nearly cylindrical, rounded at the end, and entirely destitute of spines. 

 In the smaller forms the apical (proximal) ray is usually about 300 ^ long, the 

 longest lateral ray being 220-340 n. In the large forms the longest lateral ray 

 attains a length of 650-1050 m- The basal thickness of the rays is 20-32 m in the 

 former, and 40-80 fi in the latter. 



The hexadine megascleres (Plate 53, fig. 8) have conic and blunt, usually 

 fairly straight rays. In many, two opposite rays are longer than the other four. 

 The intact hexactines ob.served were 0.6-1.6 mm. in maximum diameter, and 

 had rays 15-35 m thick at the base. Some fragmentary ones had rays as much 

 as 40 n thick. 



Among the stout-rayed tetractine and tetractine-derivate acanthophores (Plate 54, 

 figs. 1-15), which form the principal part of the skeleton of the hard superficial 

 knobs, forms occur with four, three, two, and one ray. Those with four rays, 

 that is the tetractines (or stauractines) , are the most frequent. The rays of 

 these spicules are on the whole cylindrical or cylindroconic, and rounded or, 

 more rarely, abruptly pointed at the end. They are straight or slightly curved, 

 and generally somewhat irregular in outline. The end-part of the ray is always 

 densely spined, the proximal part, usually one half to two thirds of it, is smooth. 

 Sometimes the spines extend farther proximally, occasionally quite down to the 

 centre of the spicule. The tetractines (stauractines) and triactines (tauactines) 

 (Plate 54, figs. 1-10) are 150-650 n in diameter, and have rays 10-37 m thick at 

 the base. The diactines (Plate 54, figs. 11, 12) are usually straight (Plate 54, 

 fig. 12) or more rarely abruptly rectangularly bent at the morphological centre 

 (Plate 54, fig. 11). The morphological centre is always thickened to a central 

 tyle, which is generally smooth, or more rarely spined. These spicules are 

 usually 0.4-1 mm. long, and 20-25 m thick near the middle. The central tyle 

 measures 30-50 m in diameter. The monactines (Plate 54, figs. 13-15) are usu- 

 ally 250-450 fx long, more or less cylindrical, 10-20 n thick, and closely resemble 

 single rays of the other forms. The end corresponding to the morphological 

 centre is thickened to a more or less spherical terminal tyle 18-40 m in diameter. 



The slender-rayed, long-spined acanthophores of the hard superficial knobs 

 (Plate 54, figs. 30-33, 36) are usually hexactine, pentactine, or more rarely 

 tetractine. The hexactines and pentactines form a series extending from regular 

 hexactines (Plate 54, fig. 32) to pinule-like pentactines (Plates 54, fig. 35). 

 The tetractines are to be considered as pentactine-derivates. These slender- 

 rayed spicules measure 80-180 m in diameter. Their rays are straight, joined 



