MOLLUSCA CEPHALOPODA. '239 



presents very curious phenomena. The animal, at 

 certain periods of its growth, finding itself cramped 

 in the narrow part of the spire, withdraws from it 

 that portion of the mantle which had been lodged 

 there, a vacant space being left. The surface of 

 this part of the mantle immediately begins to 

 secrete calcareous matter, which is deposited in 

 the form of a partition, stretching across the area 

 of the cavity. As the animal proceeds to increase 

 in size, and to occupy a wider portion of the ex- 

 ternal shell, the same necessity soon recurs, and 

 the same expedient is resorted to. It again with- 

 draws its mantle from the narrower into the wider 

 part of the shell ; and then forms a second par- 

 tition, at a little distance from the first, corres- 

 ponding to the space left by the receding of the 

 mantle. This process is repeated at regular in- 

 tervals, and produces the multitude of chambers 

 contained in polythalamous shells, of which the 

 living animal occupies only the largest, or that 

 which continues open ; all the other chambers 

 probably contain air only.* The partitions are in 

 general perforated either in the centre or at one 

 side, for the purpose of giving passage to a tube, 

 which extends as far as the apex of the shell. This 

 tube is often surrounded, either entirely or par- 

 tially, by shell, which forms what is denominated 

 the syphon or siphiincle ; portions of which are seen 

 in the section, Fig. 1*27. 



* This structure is exhibited in a great variety of fossil shells : 

 some of which are spiral, such as the Cornu Ammonis, while others 

 are straight cones, such as the Bacculite and Orthoceratite. In 

 most of these the partitions are very numerous, and often have undu- 

 lating surfaces. 



