MOLLUSCA CEPHALOPODA. 267 



up that portion of the mantle which occupied it, 

 thus leaving a vacant space. The surface 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 

 necessity soon recurs, and the same expedient is 

 again resorted to. It withdraws its mantle from 

 the narrower into the wider part of the shell ; 

 and then forms a second partition, at a little dis- 

 tance from the first, corresponding to the space 

 left by the receding of the mantle. This process 

 is repeated at regular intervals, and produces 

 the multitude of chambers contained in poly- 

 thalamous shells, of which the living animal oc- 

 cupies only the largest, or that which continues 

 open.* The partitions are in general perforated 

 either in the centre or at one side, for the pur- 

 pose of giving passage to a ligament, which pre- 

 serves 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 these the partitions are very numerous, and have 

 undulating surfaces. 



