146 
HEXACTINELLA MONTICULARIS. 
doubtlessly proper to the sponge, attain a length of 225-420 n. They are 
centrotyle and anisoactine, the tyle, which marks the morphological centre, 
being situated much nearer the end from which the spines diverge than the 
other. The proportion between the length of the two actines is 2 : 3 to 1 : 3. 
Close to the tyle these uncinates are usually 2-3 /x thick, the tyle itself being 
about 0.7 more in transverse diameter than the adjacent parts of the spicule. 
The spines are numerous, very oblique, and so thin that it is impossible to see 
them with ordinary light. The u. v. photographs, however, show them clearly 
enough (Plate 28 , fig. 10). I should say that these spines are scarcely thicker 
than 0.1 ii. 
The large uncinates are rare and may be foreign to the sponge. All those 
observed were broken. The largest fragments were 600-800 n long and about 5 m 
thick. Their spines are strongly inclined, nearly parallel to the shaft, and 
exceedingly thin. 
Two kinds of discohexasters, a larger and a smaller, can be distinguished. 
These are, it is true, connected by intermediate forms, but the latter are so rare 
that the distinction between them is quite clearly pronounced. 
The large discohexasters (Plate 28 , figs. 12, 15, 16, 25) measure 52-62 /x in 
total diameter, usually about 60 and have equal and regularly arranged, fairly 
smooth main-rays, 5-6 n long and about 1.8 n thick. Each main-ray bears four 
rather strongly divergent end-rays. The end-rays are curved, concave to the 
continuation of the main-ray at the base, and straight or slightly curved in an 
irregular manner farther on. The end-rays are about 23 ^ long, 1.2-1. 3 m thick 
at the base, and attenuated distally to 0.7-1 ix. They bear along their whole 
length numerous minute, backwardly directed spines and at the end a verticil 
of larger, recurved spines, which together form a kind of convex terminal disc 
with strongly serrated margin, 1.5-2. 2 /x in transverse diameter (Plate 28 , fig. 12). 
The small discohexasters (Plate 28 , figs. 11, 20) measure 30-47 n in total 
diameter, and have equal, regularly arranged, fairly smooth main-rays, 4. 5-6. 5 /j. 
long and 1-1.6 /x thick. Each main-ray bears four, exceptionally five, end-rays. 
These are curved at the base, concave to the continuation of the main-ray, and 
nearly straight farther on. In these small discohexasters the basal curvature 
usually extends farther than in the large discohexasters. The end-rays are 
9-18 m long, 0.5-1 /x thick at the base, and attenuated distally to 0.4-0. 7 /x. 
They are covered along their whole length with numerous minute, backwardly 
directed spines, and usually bear at the end a verticil of four or more larger 
recurved spines, which, when seen in profile, together appear as a convex terminal 
