406 Dr. W. Marshall on new Siliceous Sponges. 
form of the proper skeleton-spicules, but is well characterized 
by the round oscula, and especially by the presence of a dermal 
skeleton. In consequence of the presence of this dermal 
skeleton the surface of the sponge acquires a peculiar paper- 
like appearance, and at first I was inclined to regard this 
coating as consisting of fine dried mud. Microscopic exami- 
nation, however, soon taught me better. The dermal spicules 
lie irregularly (tangentially at the incurrent and excurrent 
apertures) in a tolerably thick crumbly mass of dried sub- 
stance which cannot be removed, but rather adheres very 
firmly to the underlying skeletal parts. The colour of the 
organic substance, especially of that cementing together the 
proper skeletal elements, is chestnut-brown. It is possible 
that this colour is to be accounted for by the great amount of 
iron in the Congo (almost all Africa is exceedingly rich in 
iron), which colours the surface of the stone as if with black- 
lead and penetrates several millimetres into it; but in this 
case it is certainly wonderful that the other sponges found 
there under precisely similar conditions show no trace of it. 
3. Potamolepis Pechuélit, n. sp. 
Crust-like, with numerous oscular cones as much as 10 
millim. in height, which are so compressed laterally that one 
diameter of the base is in proportion to the other as 1 to 2. 
The cones stand in the single specimen in the direction of 
their longest diameter in indistinct rows, which diverge radi- 
ately from one spot on the margin; only quite exceptionally 
do they stand perpendicularly to ie adherent base of the 
sponge, but most of them ascend with an inclination of 45° in 
the direction of the greater diameter, and in all the inclined 
side is turned towards the point from which the rows of cones 
radiate, while the other side descends steeply. At the summit 
of eel cone there is a mouth-opening (in some which are fused 
together, two), which is simply round, and may attain a dia- 
meter of 3 millim. These oscula lead into simple short gas- 
tric spaces (4 millim. deep in the longest cones), and these 
break up into a gastro-vascular system, which is not further 
traceable. The spicules present a somewhat slenderer form 
and somewhat smaller curvature than those of the other two 
species; they are cemented together by a small quantity of 
organic substance into coarse trains as much as | millim. 
broad, which, crossing each other irregularly, form a very 
confused network with meshes of very unequal size (0°5-2 
millim.) and quite dissimilar form, in some of which, very 
probably, there will during lite have been at least incurrent 
apertures. ‘The network is developed in exactly the same 
a. oh ee 
