Dr. W. Marshall on new Siliceous Sponges. 407 
manner upon the mantle of the cones as in the interspaces 
and in the interior of the sponge. The colour of the dry 
sponge is ash-grey with a silky lustre, and this and the large 
size of the meshes give it the appearance of a coarsely porous 
pumice-stone. 
With regard to the conditions under which the Potamo- 
lepides occur, their discoverer, Dr. Pechuél-Lische, has given 
me orally detailed information. The specimens come from 
above Isangila, a place which is distant from the sea about 
150 nautical miles by water, and is situated upon the Congo 
more than 100 metres above the sea-level. ‘They were also 
observed near Kalubu, about 50 nautical miles further up the 
stream. Between the place where they were found and the 
sea the river forms six talls and rapids—namely, near Yilala, 
Manguvu, Inga, Nsongo Yilala, Ngoma, and Isangila itself. In 
the actual bed of the river, on the constantly submerged rocks, 
they were not observed, which, however, may have been a mat- 
ter of chance; but the rocks of the inundation-region at the 
sides were in places thickly covered with them, so that, as the 
greater part of the sponges are white, this gave the rocks the 
appearance of being covered with the excrements of the wading 
birds which are so numerous there, as indeed my honoured 
friend at first believed to be the case. The most remarkable 
thing is that these rocks, which, during the high water of the 
summer, with its rapid flow (of about one German mile per 
hour), are from 2 to 8 metres under the surface of the water 
le, during the months of June, July, August, and half Sep- 
tember, perfectly dry under the scorching heat of an African 
tropical sun. ‘I'he sponges were collected in July. These 
rocks belong to one of the clay-slates striking from south-east 
to north-west and dipping to the south-west, and are covered 
with sponges only on their eastern side, which is towards the 
stream and more or less overhangs it, and here, which is 
sufficiently important, with all the three species together. Fre- 
quently spaces of a square metre are overgrown, but not so 
that the sponges form a connected coating ; they certainly 
stand close together, but always in distinct colonies not bigger 
than a plate. On the diabase-rocks which cross the bed of 
the river near the rapids of Isangila, no Potamolepides were 
found. ‘Their absence from these rocks need not by any means 
be due to an aversion of the sponges to this rock, and a pre- 
ference for the clay-slate ; at the spots where the diabase- 
rocks occur, and through them, the conditions of flow of the 
river will probably be so modified as to prevent any favour- 
able development of the sponge. 
