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 // 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 fi. 



The large uncinates are rare and may be foreign to the sponge. All those 

 observed were broken. The largest fragments were 600-800 ij. long and about 5 n 

 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 n in 

 total diameter, usually about 60 fi, and have equal and regularly arranged, fairly 

 smooth main-rays, 5-6 ju long and about 1.8 m 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 /i long, 1.2-1.3 m thick 

 at the base, and attenuated distally to 0.7-1 ij.. 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 m in transverse diameter (Plate 28, fig. 12). 



The small discohexasters (Plate 28, figs. 11, 20) measure 30-47 fi in total 

 diameter, and have equal, regularly arranged, fairly smooth main-rays, 4.5-0.5 ^ 

 long and 1-1.6 yu 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 

 &-18 IX long, 0.5-1 IX thick at the base, and attenuated distally to 0.4-0.7 ju. 

 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 



