96 



shutting and sealing up so much of the hinder part of the shell as it had 

 then quitted. To enable it to do this, by forming a transvere septum of 

 an appropriate form, it needed only to possess, at its .posterior termina- 

 tion, an organization calculated for the secretion, deposition, and mo- 

 delling, of shelly matter. 



The absolute weight of the animal must necessarily have increased 

 with its growth ; but if with this increase of growth an additional chamber 

 of air was produced, the animal and its appendage would preserve the 

 same degree of specific gravity. It is evident, that whilst a due proportion 

 was preserved between the solid part of the animal and these testaceous 

 air-vessels, the animal to which they were attached would not be at all 

 affected by their weight, let the number or size of those which were accu- 

 mulated on it have been ever so considerable. 



It seems to be a characterizing property of the animals belonging to some 

 ofthe shells of this genus, to close or fill up all that posterior part of the shell 

 which they do not inhabit : and in some of these we have seen, that 

 by leaving these chambers empty, the shell and animal have probably 

 preserved the same degree of buoyancy through their whole growth. But 

 in another very curious species, the Serpula heliciformis, known only in a 

 recent state, the whole dwelling of which appears to be not testaceous, 

 but actually spathous, the posterior seeming snail part is gradually filled 

 up, so as to become a solid mass of apparent spathose matter. 



To account for this difference, it only seems to be required to consider 

 that this shell does not attach itself to light bodies, whose buoyancy it 

 would affect, but that it is always found imbedded in fixed masses of 

 madrepore, and in general of Madrepora meandrites, Linn, where, from 

 the body being fixed, in which it inhabits, no regulation of its weight is 

 necessary. 



The fossil, Plate VII. Fig. 7, formed by almost horizontally disposed 

 spiral turns, flattish, but rather rounded and slightly carinated on each side, 

 the last turn being a little produced, and terminating in a round and dis- 

 tinctly marginated aperture, is, as well as the following, introduced in this 



