98 
THE VOYAGE OF H.M.S. CHALLENGER. 
number of specimens — then the Mediterranean and Arctic species of Tlienea are distinct. 
In the latter the canals are reduced to vesicles by very thick walls of collenchyme 
(PI. VII. fig. 3), in the former only some of the canals are provided with collenchy- 
matous walls, and the chambers open into wide spaces, the sinuses of folds which iu some 
cases have scarcely yet been converted into tubular canals. The part of the section 
chosen for illustration (PI. VII. fig. 4) shows more abundant collenchyme than occurs 
elsewhere in the same slice. The flagellated chambers vary from about 0‘036 by 0'044 
mm. to 0'04 by 0'06 mm. in diameter. In the slices of the Mediterranean specimens 
a thick layer of spirasters, three or four deep, characterises the outer layer of the ectosome ; 
in Tlienea muricata this is not present. 
Pcecillastra compressa (Bowerbank). 
Ecionema compressa, Bowerbank, Mon. Brit. Spong., vol. ii, p. 55, 18G6 ; vol. iii. p. 19, pi. ix. 
figs. 1-12, 1874. 
Normania crassa, Bowerbank, Brit. Assoc. Rep., p. 328, 1868 ; Mon. Brit. Spong., vol. iii. 
p. 258, pi. Ixxxi. figs. 1-12, 1874; vol. iv. p. 29, 1882. 
Hxjmenimidon placentula, Bowerbank, Mon. Brit. Spong., vol. iii. pp. 189, 353, pi. Ixxii., 1874. 
Sponge. — Irregularly plate-shaped or cup-like ; oscules conspicuous, the patent ends 
of excurrent canals, irregularly distributed over one face of the sponge, pores evenly 
dispersed over the other. 
Skeleton as in Poecillastra schulzii. 
Spicules.- — ^I. Megascleres. 1. Oxea, fusiform, 1’6 to 1'9 by 0‘03 to 0’045 mm. 
2. Orthotrisene, cladi varying from about 0T7 to 0'32 mm. by 0‘25 mm. ; one or two 
cladi frequently suppressed. This spicule passes into a calthrops in the interior of the 
sponge. 
II. Microscleres. 3. Microxea, 0T4 to 0’2 mm. long. 4. Plesiaster, a single actine 
0’028 mm. long. 5. Spiraster, 0'02 mm. long. 
Colour. — Light grey. 
Habitat . — Shetland, 110 fathoms; west of Scotland and Hebrides; Queen Charlotte 
Islands, North Am,erica. 
Remarks . — Type specimens of Bowerbank’s three species were kindly lent me by Dr. 
Norman, and, after a careful comparison, I can find no difference between them. Mr. 
Carter’s assertion of their identity must therefore be accepted. At first I thought I had 
discovered a difference in the oscules, but closer examination shows that they have the 
same characters in all — those, namely, so well portrayed in Bowerbank’s figure of Nor- 
mania crassa. Since the specific name “ compressa ” has priority, we have no choice but 
to adopt it, though it is not very applicable as a descriptive term to all the individuals 
of the species. The additional locality (Queen Charlotte Islands) rests upon my deter- 
