Mr. Parkinson o« Hlppuntes. 281 



spoken only of as lateral ridges passing through the chambers and 

 septa, they will be found in most specimens to be formed of a solid 

 spathose substance. But in the specimen No. VI, in which is a 

 transverse section of one of these shells near to its upper extremity, 

 these ridges will be seen, in consequence of the cavities not having 

 beenjfilled by the infiltration of calcareous matter, to have originally 

 possessed a singular organization : a substance of extremely loose 

 and light texture is disposed in plates, which radiate towards the 

 sides of the containing tubes, leaving interstices between them, nearly 

 equal in size to that of the radiating substance itself. 



It is indeed impossible to speak decidedly as to the manner in which 

 this peculiar organization could influence the buoyancy of the animal 

 and its dwelling. It is however not difficult to conceive that the 

 animal might possess the power of contracting and of expanding 

 this radiating substance ; and might, by thus admitting or expelling 

 the water, produce that change in the specific gravity of the whole 

 mass as might occasion it to sink or rise according as the necessities 

 of the animal might direct. 



The great substance of this shell, it being generally about half an 

 inch in thickness, does not, it must be acknowledged, appear at 

 first sight to be favourable to the opinion of its having been capable 

 of being rendered buoyant. But the appearances offered to our ob- 

 servation by the specimen No. VII. give reason for supposing that 

 the texture of the shell was such, that its size might rather promote 

 than prevent its buoyancy. In this specimen the looseness of the 

 texture of the shell is very remarkable ; it must however be admitted, 

 that it is difficult to ascertain how much of this depends on the 

 original lightness of structure, the interstices not having been filled 

 up by calcareous matter, and how much on subsequent decom- 

 position. 



Vol. II. 2 n 



