406 On a new Genus and Species of Renierid Sponge. 
usually exhibit the internal skeleton uncovered, either as a 
projecting tuft of frayed fibres, or as forming the sides of a 
conical cavity excavated axially from the end of the branch 
inwards. 
The absence of distinct oscules and aquiferous canals, toge¬ 
ther with the incomplete closure of the ends of the branches 
and the greater openness of the internal skeleton along lines 
radiating from the centre of the branches obliquely forwards 
and outwards, lead one to conjecture whether the external 
water, after finding access to the interior of the sponge through 
the pores of the dermal layer, may not have found its way 
outwards along the lines of least obstruction in the skeleton, 
and finally have discharged itself by the more or less open 
ends of the branches. This, however, is merely conjecture; 
and without actual observation of the sponge in the living- 
state one cannot expect the nature of its water-circulation to 
be made clear. 
The variations in form of the spicules are chiefly manifested 
in the character of their terminations. The somewhat sharp 
points of fig. 5 are the most usual; but very frequently the 
termination is more abrupt, and we have the conical form of 
fig. 12; this readily passes into the shouldered and pointed end 
of fig. 11, a very common form of termination, which, by 
losing its mucrone, passes into the rounded-off ends of the 
rare form (fig. 14). The tendency of these variations appears 
to be in the direction of lateral development towards the point, 
which, in its most exaggerated form, is shown by the pin¬ 
headed spicule of fig. 13, of which a few instances have been 
observed. 
Budding occurs occasionally, as in fig. 10, where the small 
offshoot on the left side of the spicule is mucronote, like the 
normal point of the spicule, though the latter, in this case, is 
twice shouldered, or diminishes to a point by two stages. 
There is an obvious resemblance between this recent sponge 
and the extinct Pharetrospongia of the Cambridge Greensand: 
the thickness of the plate-like wall in the one is very nearly 
the same as that of the palmate expansion of the other; both 
possess a spiculo-fibrous reticulate skeleton; both are distin¬ 
guished by the inconspicuous character of their oscules and 
excretory canals ; both agree in the form and size of their 
spicular elements ; and the only marked differences which 
distinguish them are to be found in the branched form of the 
recent sponge, and the absence of flesh-spicules in the fossil 
one. The chance of flesh-spicules being preserved in the 
fossil state is so remote, however, that Pliaretrospongia , if it 
originally possessed them, would certainly betray no signs of 
