Siliceous Spicules of Sponges. 209 



Schmidt figures several spicules of this form (t. iii. f. 28, 

 29, t. iv. f. 2, 7, &c), and Bowerbank (P. Z. S. 1872, t. v. 

 f. x. & xi., t. xlvi. & xlvii. ; 1873, t. i.-iii. & vii. 



Sometimes one of the lateral rays (B. S. t. ii. f. 48), more fre- 

 quently all, especially when expanded, are forked at the end 

 (t. ii. f. 50, 51, called " furcated attenuato-patento-ternate spi- 

 cules," and t. v. f. 130, where they are called " bifurcated 

 expando-ternate "). The difference between the two spicules 

 figured is that in f. 50 & 51 the rays are expanded horizontally, 

 and in f. 130 they are directed rather backwards from the axis. 



Schmidt figures this kind of spicule at t. iv. f. 5 ; but he 

 also figures (Suppl. t. iii. f. 3), with the forks well defined, 

 spicules with only short recurved rays — one, two, or three of 

 the rays being bifid at the end — showing how these forms pass 

 into one another, even in the same sponge. 



Bowerbank figures (t. xxxi. f. 362) a portion of Tethea 

 cranium, in which he shows the exceeding difference of form 

 of the spicules of this group which are found in a single frag- 

 ment of a single species. If carefully examined, in it will be 

 found almost all the forms of trifurcate spicule with simple 

 rays I have noticed above, including those with the central 

 axis produced beyond the rays, the rays bent from or recurved 

 towards the axis of the same small slice of a sponge. 



The " verticillately spined" triradiate spicule (B. S. t. iv. 

 f. 84, t. x. f. 235, 236) appears to belong to this type, with 

 its axis reduced to a minimum on each side. 



The quinqueradiate spicules generally have an elongated 

 axis compared with their lateral rays ; but in one modification 

 the axis is not produced on the outer side, and is very short 

 and rudimentary on the other side; and the bifurcate rays 

 are very broad and spreading, like the bifurcate rays of the 

 more typical spicules of the series (B. S. t. ii. f. 52, the irre- 

 gular furcated patento-ternate spicules, and t. ii. f. 53, which 

 has the axis slightly developed on both sides of the rays). 

 Spicules of this form have sometimes on one side a very 

 short axis, and a broad, expanded, terminal disk, which is 

 sometimes orbicular, nearly entire, and at others crenated or 

 divided into lobes on the edge. When lobed it is usually 

 divided into three greater portions, which are lobed on their 

 surface ; but whatever may be the form of the disk, they are 

 always marked with three short lines diverging from the 

 central spot, and sometimes forked at the end (B. S. t. iv. 

 f. 102, 103, t. v. f. 104-108), and are called " foliato-peltate." 

 Carter, Ann. & Mag. N. H. 1871, vii. t. vii. & viii. These 

 spicules occur in the different species of Coralloid or Lithisti- 

 dine sponges, and show the existence of quinqueradiate 

 spicules in that family, as the spherules which are character- 



