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 shagreeu, 

 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, limestonCj 

 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 Cliona is 

 a mere circular speck just visible to the naked eye ; PI. XIV. 

 fig. 4 «, 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, b ; 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 increase in thickness; numerous papilla?,/, being added, 

 which penetrate the surface of the shell, and the terminations of 



