208 HYALONEMA (HYALONEMA) PLACUNA. 
specimens differ in detail to such an extent that I have established two distinct 
forms, A and B. 
Shape and size. The specimen of form A is the better preserved. This 
sponge (Plate 64, fig. 11) appears as a thin, irregularly oval lamella with a slight 
marginal protuberance at one of the narrow ends. Part of its margin is torn 
off. The sponge is 65 mm. broad and, together with the protuberance, 80 mm. 
long. A number of transverse folds slightly protrude from its surface. These 
folds are strongly inclined towards what appears to be the upper end of the 
sponge, and are here more numerous and crowded than below. Their margins 
form more or less concentric curves, which are convex towards the upper end of 
the sponge and extend across the whole lamella. These folds, which are much 
more clearly marked on one face of the lamella than on the other, give to the 
sponge its Placuna-like appearance. 
The sponge is not, as at first sight appears, a simple plate, but is composed 
of two lamellae, 1.5-3 mm. thick, closely pressed together and joined along one 
side. In life it was probably a thin-walled sac, and I am inclined to ascribe its 
present lamellar shape to a compression and complete flattening after capture. 
The intact parts of the free margin of this sac bear a frill of freely projecting 
spicules. The protuberance (Plate 64, fig. 11) is part of this marginal frill. 
The outer surface appears rough and exhibits the folds mentioned above. Aper- 
tures (pores) were not found in it. The inner surface is smooth, and also bears 
a few strongly inclined projecting folds. 
The specimen of form B is similar (Plate 64, fig. 12), but more fragmentary. 
It appears as a lamella, about 3 mm. thick, with somewhat irregular outline, and 
is 65 mm. long and 42 mm. broad. 
The colour of both specimens in spirit is dirty white. 
Skeleton. The outer surface is covered with a dense fur of large dermal 
pinules (Plate 64, fig. 18a; Plate 65, figs. 22,23). Diactine pinules and centro- 
tyle amphioxes protrude from the sharp margin of the probably sac-shaped body. 
These pinules form the marginal frill referred to. The inner surface like the outer 
bears pinules. These gastral pinules are smaller, scarcer, and not nearly so 
densely crowded as the dermal. In form B two kinds of internal pinules can be 
distinguished, a larger with long lateral rays, and a smaller with apparently 
rudimentary lateral rays. The former are certainly gastral, the latter may be 
canaliculate. Megascleres are very abundant just below the surface and in 
the interior rhabds. Most of them are centrotyle amphioxes of moderate 
thickness; some are short spindle-shaped centrotyle amphioxes with remarkably 
