192 THE MECHANICAL FUNCTIONS. 



row part of the spire, draws up that portion of the mantle 

 which occupied it, thus leaving a vacant space. The sur- 

 face of the mantle which has receded, immediately begins 

 to secrete calcareous matter, which is deposited in the form 

 of a partition, stretching completely across the area of the 

 cavity. As the animal proceeds to increase in size, and to 

 occupy a wider portion of the external shell, the same ne- 

 cessity soon recurs, and the same expedient is again resort- 

 ed to. It withdraws its mantle from the narrower into the 

 wider part of the sheH; and then forms a second partition, 

 at a little distance from the first, corresponding to the space 

 left by the receding of the mantle. This process is repeat- 

 ed at regular intervals, and produces the multitude of cham- 

 bers contained in polythalamous shells, of which the living 

 animal occupies only the largest, or that which continues 

 open.* The partitions are in general perforated either in the 

 centre or at one side, for the purpose of giving passage to a 

 ligament, which preserves the attachment of the mantle to 

 the apex of the shell. This ligament is often surrounded 

 either entirely or partially by shell, which forms a tube, 

 denominated the syphon: and portions of which are seen in 

 the section Fig. 127. 



* This structure is extremely prevalent in 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 tliese the partitions are very nu- 

 merous, and have undulating- surfaces. 



