100 



THE GEOLOGIST. 



respecting Sepia, Spii^ula and the Belemnites, these Septa, 

 are used for the purpose of retaining air, and are des- 

 tined to sustain the animal, to render it lighter in the 

 water, and facihtate its progression ; thus we see the number 

 of chambers, formed by the Septa, augment as the animal 

 increases in size, so as always to compensate for its in- 

 creasing weight, and maintain a perfect equilibrium, at all 

 periods of its existence, between the animal and its shell. 



Fom of the Septa. — The edges of the Septa, in the Am- 

 monites, are formed of lobes and saddles, (see figs. 1 and 2.) 



Fig. 1. 



(2) The following letters which are the same as those employed by 



