174 HYALONEMA (HYALONEMA) AGASSIZI. 
broken off 1-3 em. from the sponge, arise from the centre of the flat lower face. 
Many parts of the surface, particularly of the upper side, exhibit a reticulate 
appearance, caused by the presence of a superficial network with irregularly 
square meshes, the centres of which are about 1 mm. apart (Plate 41, fig. 14). 
The centre of each mesh is occupied by the circular entrance to an afferent 
canal. The other two specimens of this form are similar. One of them is about 
as long and broad as the one above described, but thicker (higher); the distance 
between the summit of its gastral cone and the slight protuberance on its lower 
face, from which the (absent) stalk arose, being 40 mm. The other is smaller, 
has no trace of a stalk, and measures 45 by 37 by 12 mm. 
The single specimen of form D is also indifferently preserved. A few small 
remnants of the dermal membrane and some stalk-spicules, broken off short, 
are, however, still present. This sponge (Plate 41, fig. 12) is also cake-shaped. 
It measures 51 mm. in length, 46 mm. in breadth, and 17 mm. in thickness 
(height). 
The larger and more complete of the two specimens of form E is cake- 
shaped, 36 mm. long, 28 mm. broad, and 25 mm. thick (high). In the middle of 
its flattened upper face there is a gastral depression, surrounded by a thin circu- 
lar wall with sharp margin. This margin is nearly circular and measures 13 mm. 
in diameter. In the centre of the depression a low gastral cone is situated, from 
which radiate several somewhat irregularly disposed vertical plates. Between 
these plates wide diverticula of the gastral cavity extend downwards to a dis- 
tance of about 14 mm. A protuberance 5 mm. high is observed in the middle 
of the lower, more convex face of the sponge. The holes in it indicate that, 
in life, the spicules forming the stalk arose from this protuberance. The other 
specimen of this form is very similar. It measures 34 by 28 by 21 mm. 
The most complete specimen of form F is laterally compressed and appears 
as an irregularly triangular plate about 5mm. thick. The plate is 30 mm. broad 
above and narrows below to 5mm. A bundle of stalk-spicules arises from the 
lower end. The other two specimens of this form are more fragmentary and 
consist only of the central and the attenuated basal part of the sponge-body, 
and the upper part of the stalk. One is (without the stalk) 37 mm. long, the 
other 35 mm. The largest of the fragments of this form is 28 mm. long. 
The colour of the specimen of form A in spirit is a rather rich coffee-brown, 
of form B a dirty light greenish gray, of form C a light reddish brown, of form 
D a light dirty brown, and forms E and F are whitish. 
Canal-system. The canal-system of form A (Plate 45, figs. 18, 23) seems to 
