HYALONEMA (HYALOXEMA) OBTUSUM. 16l 



long, straight, conic, rather blunt-pointed, and 7-35 /u thick at the base. Their 

 basal thickness is roughly speaking in proportion to their length. 



Very young stages of these hexactines appear as spheres, 20 ix in diameter, 

 perforated by six axial cylinder threads, 5 m thick, which are joined at right 

 angles in the centre. Wliere these axial cylinder threads reach the surface of 

 the sphere this is elevated in the shape of very thin-walled tubes rising about 

 10 II over the surface of the sphere (Plate 39, fig. 5). 



The hexadine megascleres of the loose axial spicular column, which were found 

 only in var. gracilis, appear to be larger than the more superficially situated, but 

 since I have not been able to find any intact ones, I can only say that their 

 longitudinally extenchng rays appear to be much longer than their transverse 

 rays, and that their rays are, at the base, about 40 ^ thick. 



The stout acanthophores (Plate 36, figs. 1-25, 27-45; Plate 39, figs. 17-21, 

 34-38) of the basal part of the sponge-body range from pentactine to monactine. 



The peritactines are rare. The few observed in var. gracilis were 225-530 m 

 in diameter, and had rays, at the base, 12-29 ii thick. 



The tetractines (Plate 36, figs. 1-25, 27, 28; Plate 39, figs. 18-20) generally 

 have more or less unequal rays. The inequality of the rays is often very con- 

 siderable. The rays are exceedingly variable in size, curvature, shape, and 

 spinulation, but constant and uniform in so far as their basal parts always form 

 a fairly regular, rectangular cross, and as the rays themselves always appear to 

 extend nearly in one plane. The tips of the rays are nearly always more or less 

 spiny, only quite exceptionally (Plate 39, fig. 20) entirely smooth. In both 

 varieties these spicules measure 180-840 /x in total diameter. Among the irregu- 

 lar ones all sizes between these limits are met. The regular ones never appear 

 to exceed 500 ^ in diameter. The rays are generally wavy in outline, cylindro- 

 conical or cylindrical, and chstally thickened, or, more rarely, without a thick- 

 ening at or near the end (Plate 36, fig. 1; Plate 39, fig. 20). The ray either 

 terminates with the distal thickening and then appears simply rounded off at the 

 end (Plate 36, figs. 22, 23, 25; Plate 39, fig. 18), or it is continued beyond the distal 

 thickening in the shape of a terminal cone (Plate 36, fig. 7). The rays of these 

 spicules are in var. gracilis 35-380 fi long and 12-35 fi thick at the base; in var. 

 robusta, where they are more irregular and stouter, 40-500 n long and, at the base, 

 20-50 n thick. The distal thickening is in the tetractines of var. gracilis 10-40 n 

 in chameter, in those of var. robusta 10-60 /u. 



The thickness of the rays is not in proportion to their length, and varies in 

 the rays of all lengths between similar limits. We consequently find among the 



