144 THE VOYAGE OF H.M.S. CHALLENGER. 



spicula," Bk.), but more spined at the base than elsewhere ; usually curved, finely 

 pointed; verging upon tylostylote ; size up to about 0'2 by 0"0084 mm. (3) Very 

 slender tylota, with very slightly developed heads, not spined nor flattened at the ends ; 

 size up to about 0"32 by 0"0047 mm. (4) Tridentate isochelse,^ with strongly curved 

 shaft and divergent teeth ; length about 0"04 mm. 



The 'points in which Bowerbank's species difi"ers from Myxilla nohilis will be readily 

 seen by comparison of the descriptions ; they concern more especially the form of the 

 larger stylus and of the tylote spicule.^ 



Myxilla froridosa, Ridley and Dendy (PI. XXVI figs. 1, la; PI. XXVII. figs. 10, 

 10a, 106, 10c, lOfZ, lOe, 10/). 



1886. Myxilla frondosa, Ridley and Dendy, Ann. and Mag. Nat. Hist., ser. 5, vol. xviii. p. 473. 



Sponge (PI. XXVI. figs. 1, la) represented by a single broad, flattened frond, of some- 

 what oval shape, about 6 mm. thick, 100 mm. in height and 62 mm. in breadth. It 

 has evidently grown in an erect position. Colour in spirit rather dark, yellowish-grey. 

 Texture tough, fibrous, elastic. Surface, on one side, which is slightly convex, very 

 rough, owing to tlie presence of thickly placed, small conuli, between which are the oscula. 

 The other side is much smoother, the conuli are not so strongly pronounced, and the 

 dermal membrane stretches over them continuously except where it appears to have been 

 rubbed off". Dermal inembrane distinct, thin and transparent. Oscula very numerous, 

 small, round and thickly scattered over the convex surface, to which they appear to be 

 confined ; about 1 mm. in diameter. Pores very numerous, scattered, rounded openings 

 in the dermal membrane on the concave surface, diameter variable, averaging about 

 0*2 mm. We have seen no pores on the other surface of the sponge. 



Skeleton. — (a) Dermal; consisting of tylote spicules, which are for the most 

 part irregularly scattered through the dermal membrane but are sometimes found in 

 loose tufts. (6) Main ; a very highly developed and compact reticulation of coarse, 

 branching and sometimes anastomosing spiculo-fibre. The main lines of the skeleton, 

 starting from the base, branch upwards in a dendroid manner, with their ramifications 

 lying mainly in one plane, and giving ofi", approximately at right angles to this plane, 

 very numerous, short branches which end in the conuli on the surface of the sponge. The 

 fibre itself is stout and consists of closely placed styli which have their bases in the 

 centre of the fibre and their apices directed very obliquely outwards and free, point- 

 ing towards the direction in which the fibre is running ; there seems to be very little 

 cementing substance uniting the spicules. 



S2ncules. — (a) Megasclera ; (l) Entirely, but only slightly spined styli (PI. XXVII. 



' The figure of this spicule given by Bowerbank (Mon. Brit. Spong., vol. iii., pi. xxxvii. fig. 8) appears to us to 

 convey a very erroneous idea of its true shape. 



^ Cf. Bk., Mon. Brit. Spong., vol. iii., pi. xx,\vii. figs. 5, 6. 



