768 MR. A. G. THACKER ON [June 16, 



The tubar skeleton consists almost exclusively of triradiate 

 spicules. The majority of these spicules are markedly sagittal 

 but there is much variation in this respect, some spicules being 

 almost equiangular and equiradiate, whilst others have an angle 

 amounting nearly to 180° and the other two angles only a little 

 more than 90° each. In typical spicules the sagittal form is due 

 both to this difierence of angles and to the greater length which 

 is attained by the basal ray. All the rays are usually straight ; 

 but sometimes the paired rays are slightly curved throughout 

 their lengths, in which case they always have their convex sides, 

 never the concave, towards the basal ray ; and sometimes the 

 basal ray is curved at its extremity. In a few cases the basal ray 

 is shorter than the paired rays. The size of an avei-age triradiate 

 of the tubar skeleton is as follows : — Length of paired rays, 

 0*09 mm. ; width at base of rays, 0'012 mm. Length of basal 

 ray, 0"11 mm. ; width at base, 0*012 mm. The great majority of 

 the spicules are of this size or very close to it, but the rays are 

 occasionally considerably longer, although never much wider. 

 A few quadriradiate spicules, with facial rays resembling the rays 

 of the triradiates and with shoi-t apical rays, occur in the proximal 

 portions of the radial canals ; they are, however, very scarce and 

 are probably of little systematic importance. 



The crowning tufts at the ends of the radial chambers are 

 composed of oxea of thi'ee sizes. The largest of these (text-fig. 

 161, b) reach a length of 0'8 mm. and a maximum thickness of 

 0'05 mm. They are usually sharply pointed at both ends, but 

 they taper to the point more gradually at one (the projecting) end 

 than at the other, the thickest part of the spicule not being in the 

 middle of its length ; this character is very marked in some 

 spicviles, but in others it is scarcely noticeable. These spicules 

 are somewhat flattened, are straight or nearly straight, and have 

 about one-third of their lengths projecting beyond the surface of 

 the sponge. The tufts also consist in part of much smaller straight 

 or somewhat curved, sharply pointed oxea (text-fig. 161, c), of a 

 diameter only about equal to that of the rays of the triradiates ; 

 these reach a length of 0*25 mm. and have a maximum thickness 

 of 0"01 mm. In addition to these there are a number of very fine 

 haii'-like oxea which may perhaps be young spicules. 



The gastral cortex, which is well developed and is four or five 

 layers of spicules thick, consists of triradiates and quadriradiates, 

 the apical rays of the latter projecting freely into the gastral 

 cavity. As in the tubar skeleton, there exists considerable varia- 

 tion in the spicules ; the triradiates are typically somewhat longer 

 and narrower than those of the radial canals, but otherwise there 

 is little difference between the triradiates of the two regions. 

 The average length of the rays is 0"12 mm., and their width at 

 base 0"008 mm. One angle is frequently greater than the other 

 two, thus making the spicules sagittal. The facial rays of the 

 . quadriradiates resemble the rays of the triradiates ; the apical 

 rays are short and stout and sharply pointed ; they are either 



