Prof. W. Thomson on the “Vitreous” Sponges. 181 
The only known specimens of Habrodictyon corbicula are in 
the Museum of the Jardin des Plantes. They are three in 
number: one is perfect, another is torn through the middle, 
and the third is a mere fragment. Through the friendly cour- 
tesy of M. Lacaze-Duthiers, I had an opportunity of examining 
them carefully; and, by his permission, an admirable photo- 
graph of the perfect specimen was taken for me by M. Potteau. 
This photograph is copied, reduced one-third in size, in Pl. IV. 
fig. 1. The specimen is labelled ‘“ Aleyoncellum corbieula, Val. 
Donné par M. Saches. 1857.” 
H. speciosum, Quoy & Gaimard (sp.). Pl. IV. fig. 2. 
ooo Sa speciosum, Quoy & Gaimard, ‘ Voyage de l’Astrolabe’, Zoologie, 
vol. iv. p. 302. 
Alcyoncellum speciosum, Milne-Edwards, in Lamarck’s ‘Animaux sans 
Vertébres, vol. ii. p. 589. 
Alcyonellum gelatinosum and A, speciosum, Owen, Trans. Zool. Soc. vol. ii. 
. 205. 
Secacalinn corbicula, Bowerbank, British Spongiade, vol. i. p. 174. 
Euplectella speciosa, Gray, Ann. & Mag. Nat. Hist. ser. 3. vol. xviii. p. 467. 
Corbitella speciosa, Gray, Proc. Zool. Soe, for 1867 , p. 530, 
The Sponge is tubular, about 7} inches in height, expand- 
ing gradually upwards from a contracted base 1} inch in dia- 
meter to a width of about 24 inches at the upper extremity. 
The network forming the wall of the tube is the same in ge- 
neral structure and arrangement as in H. corbicula,. but the 
fibres are finer and more defined and compact in texture. The 
wall of the tube rises to no definite lip, and forms no terminal 
lid, but the ordinary network of the wall simply arches over 
and closes the wide end of the tube without any change of 
structure. As a rule, the spicules of the skeleton are identical 
in form and arrangement with those of H. corbicula; the fusi- 
form spicules seem to be somewhat longer, and the irregular 
-hexradiate spicules less abundant. All the sarcode-spicules of 
H. corbicula are repeated in H. speciosum in nearly the same 
- numerical proportions; but in the latter species a minute spi- 
cule (Pl. IV. fig. 2 a, x 1000) occurs in great abundance, while 
it is rare, if it occur at all, in H. corbicula. This is probably 
Bowerbank’s “bifurcate rectangulated hexradiate spicule” 
(British Sponges, fig. 188) ; it recalls in general character and 
physiognomy a small spicule very abundant in the sarcode ot 
yalonema, figured in the ‘ Intellectual Observer’ of March 
last, plate 1. fig. 10. The abundance of this special spicule in 
H. speciosum has chiefly weighed with me in regarding the 
two forms of Habrodictyon as distinct species’; it would be ne- 
cessary, however, to examine a larger series to arrive at a de- 
finite conclusion on this point. 
