HYALONEMA (HYALONEMA) OBTUSUM. 161 
long, straight, conic, rather blunt-pointed, and 7-35 y» 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 » in diameter, 
perforated by six axial cylinder threads, 5 » thick, which are joined at right 
angles in the centre. Where these axial cylinder threads reach the surface of 
the sphere this is elevated in the shape of very thin-walled tubes rising about 
10 uw over the surface of the sphere (Plate 39, fig. 5). 
The hexactine 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 extending rays appear to be much longer than their transverse 
rays, and that their rays are, at the base, about 40 u» 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 pentactines are rare. The few observed in var. gracilis were 225-530 u 
in diameter, and had rays, at the base, 12—29 uw 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 » in total diameter. Among the irregu- 
lar ones all sizes between these limits are met. The regular ones never appear 
to exceed 500 u in diameter. The rays are generally wavy in outline, eylindro- 
conical or cylindrical, and distally 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 » long and 12-85 » thick at the base; in var. 
robusta, where they are more irregular and stouter, 40-500 » long and, at the base, 
20-50 » thick. The distal thickening isin the tetractines of var. gracilis 10-40 u 
in diameter, 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 
