322 PEOF. P. M. DUNCAN ON A 



spicula are then either doubled in number or are greatly en- 

 larged radially. Some have the limbs by which they usually unite 

 to the shorter spicula of the concentric layers greatly enlarged 

 and curved, so as to include more or less of the side of a tube 

 (Plate XXIV. fig. 3). Near the larger canals in the interior, 

 the concentric and radial arrangements are lost, and a confused 

 tissue, consisting of medium-sized spicula uniting by very spinu- 

 lose and ragged ends, exists. The bunched masses of spicular 

 endings give an opacity which is very difficult to overcome (Plate 

 XXIV. figs. 4-7). In portions of the section, the columns are 

 so thick as to render the distinction into concentric and radial 

 parts uncertain. 



The spicula of the interior of the body vary much in size, and 

 are, as a rule, beautifully transparent. The trifid outline pre- 

 vails, and the fourth limb usually exists, but sometimes it does 

 not, or is invisible. 



Curved limbs ending in ragged, semi-spinose, or flat processes 

 are common ; but this raggedness is not long or in excess. It 

 is often toned down into a series of short blunt projections, which 

 are rather opaque and reflect light strongly. 



In the columns, one limb of the spicule is always greatly deve- 

 loped, being long and narrowest centrally ; and the other limbs 

 are small, being sometimes indistinguishable from the irregular 

 junction part. 



The articular processes at the ends of the limbs are excessively 

 irregular in shape, not very close, and rather short as a, rule. 

 Projections occur on all sides of the ending limbs ; but they are 

 rare on the main one (Plate XXIV. fig. 3), where isolated spinules 

 are not uncommon. The spicula which join on to the ends of 

 these radial ones are more compact, and their limbs are more 

 equal ; they are smaller than the others, and some are minute, 

 forming an excessively close areolation. In the first instance the 

 curvatures of the spicula are very decided, and the ends of the 

 limbs are ragged (Plate XXIV. fig. 4) ; their surface is sparsely 

 ornamented with dot-like projections, circular in outline. In the 

 second case, the minute spicula run into each other at the limb- 

 ends, with or without ragged projections ; and the various angles 

 of junction produce an incoherent arrangement (Plate XXIV. 

 figs. 4-6). 



These smaller spicula are trifid and quadrifid. Where sepa- 

 rable, they present fractures and nodular points at the edge of the 



