102 



oii, ; in -a similar manner, through all the compartments; it appearing 

 to be a continued membrane, beginning with the animal, and extend- 

 ing to the first and smallest compartment; the end of each testaceous 

 tube seeming to be included in a duplicature of this membrane, and 

 placed on its outside, somewhat in the manner in which the abdo- 

 minal viscera are involved in the duplicatures on the outside of the peri- 

 toneum. 



We have thus, I trust, by the fossil remains of this genus, obtained a 

 confirmation of the opinion of Hooke, and established the fact of a con- 

 tinued tube, capable of dilatation and contraction, passing through all 

 the chambers of the shell. From what source the gaseous matter is de- 

 rived with which this tube was filled, and in what manner the animal 

 effected those modifications of the tube and its contained air, on which 

 the variation of its buoyancy depended, are subjects of inquiry still de- 

 manding the assiduous attention of the naturalist, and on which I will 

 not pretend to hazard a conjecture. 



The power of raising or sinking the shell appears, from the observa- 

 tions just made on the siphuncular membrane, to depend on this organ 

 alone : some other use remains, therefore, to be found for the closed 

 cavities of the chambers. With these it is observable, that the animal 

 preserves no communication, except for the passage of the siphunculus; 

 he closing each chamber, and completely excluding himself from them, 

 as he extends the siphunculus, and, as agreeable to the increase of his 

 growth, he forms himself a new dwelling. Hence it appears, I think* 

 highly probable, that the only use of the vacuities formed by these nu- 

 merous chambers, is to counteract the weight of the increasing mass of 

 the animal, and of the thick shell ; and thereby to render the whole so 

 nearly of the weight of the water, that the difference arising from the 

 siphuncular membrane being contracted or dilated, may occasion the 

 mass to swim or sink. It will, I trust, appear, in confirmation of this 

 opinion, that in another genus of the multilocular shells, the belemnite, 



