408 Dr. W. Marshall on new Stliceous Sponges. 
In habit the Potamolepides show no resemblance to the 
Spongille, or indeed to any Renierid, and in their firmness 
and brittleness they much rather resemble certain Hexac- 
tinellide, especially £. Pechuéli’, which, in the microscopic 
structure of its skeleton, in the arrangement of its fibres, &e., 
greatly reminds one of a Farrea. I believe these remarkable 
peculiarities will become intelligible if we consider more 
closely the conditions under which the Potamolepides live. 
Here it is to be remembered above all that they live in run- 
ning water, which certainly during the rainy season flows 
strongly, and will break with violence against the slabs of 
rock, and that they occur in opposition to the direction of the 
stream. In the presence of such factors a young sponge will 
not be able to grow into a branched shrub, or even into a 
turf-like cushion, in the same way as Spongilla lacustris or 
many Lubomirskie in still water: the pressure of the flowing 
water will rather compel it to cover its base of attachment with 
as thin a crust as possible, by which means a further advan- 
tage is gained towards its comfortable maintenance; the water 
surrounding it may indeed, as a destructive torrent, be rich in 
suitable nourishment, but it is too rapid in its movements and 
will leave the detritus contained in it for too short a time in 
one place to allow the sponge to derive much advantage from 
it, although indeed the chances may be somewhat more favour- 
able on the overhanging side of a slab of rock than elsewhere 
in the bed of the river. The sponges take up their nourish- 
ment through apertures of the surface: when it is uniformly 
abundant and easy to obtain, the sponges may be cylindrical 
or conical, which, according to their ontogeny, seems to be 
their original form, and they need not adapt themselves to an 
increased reception of food. ‘Thus in a round sponge, if we 
assume that the incurrent apertures are, under all circum- 
stances, equally distributed, the proportion of the surface of 
the body (square) to the contents of the body (cube) suffices 
to nourish it, that is to maintain it and enable it to increase 
in size and reproductive products. ‘I'he conditions are differ- 
ent if the food is scanty or difficult to obtain: then the above 
approximate proportion of the square to the cube will no 
longer suffice; the surface must be increased in proportion to 
the mass of the body, and with this the number of the food- 
receiving incurrent apertures must be increased. How can 
this be effected ? Sometimes by the formation of pits, folds, 
intercanalicular spaces, pseudogastres, &c.; but this does not 
seem to be always adinissible : residence on the underside of 
stones which do not form much of a hollow will put a veto 
upon it 5 but especially, as in the present case, very rough 
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