with descriptions of new Species. 327 
when the burrows of the two interfere with each other, which 
not unfrequently occurs. 
There is, however, a very certain character which never fails 
to determine the habitation of the burrowing sponge, even though 
every particle of the animal be removed. If the parietes of the 
chambers and ramifications are viewed through an ordinary lens, 
they are found to be distinctly punctured in a peculiar manner, 
resembling what might be supposed to be the impress of shagreen, 
only much more minute. In some species this puncturing is 
much finer than in others, and occasionally it varies a little in 
character ; but is always to be observed on the walls of the 
burrows of these sponges, whether they be in shell, limestone, 
coral, or nullipore. This puncturing therefore cannot be caused 
by the structure of the material in which the chambers are 
excavated, but must result from the character of the surface of 
their inhabitant. So certain a test is this, that by it alone the 
nature of the excavations in fossil shells may be determined 
with the greatest confidence. No other excavation, whether 
of worm or mollusk, presents a surface anything like that of 
the burrows of these sponges. And were no other proof at 
hand, this puncturing would be sufficient to establish the fact 
that these sponges possess the power of enlarging their habita- 
tion; but when taken in connexion with what has already been 
said, little doubt can exist of the fact that Cliona entirely exca- 
vates its abode: indeed after an examination of the form of these 
beings, and of the branched, lobed and systematic cavities they 
occupy, it would seem impossible to arrive at any other conclu- 
sion. On this point, however, I possess, if possible, still stronger 
evidence. 
Through the kindness of Mr. Fryer I have had the examina- 
tion of an individual of Placuna placenta, in the shell of which 
there are imbedded numerous specimens of a very beautiful 
Cliona exhibiting every stage of development from the earliest to 
maturity. This shell is so transparent, that even the minutest twigs 
are seen with the greatest precision. At first the young Clionais 
a mere circular speck just visible to the naked eye; Pl. XIV. 
fig. 4 a, represents it in this stage sunk within the substance of 
the shell, through which there is a papillary puncture almost as 
large as the individual itself: afterwards the circle increases in 
size around the papilla, and becomes irregular in form, 4; a thin 
linear branch is then pushed out from one side, c, and throws 
up through the shell another papilla. A branch from the op- 
posite side now makes its appearance, d; a third and a fourth 
succeed, e, f; these are now seen to divide gradually into lobes, 
and to imerease in thickness ; numerous papille, f, being added, 
which penetrate the surface of the shell, and the terminations of 
