246 DISTANCES OF TKANSVERSE PLATES, 



tion to the successive stages of growth of the outer shell, 

 mauitains its efficacy as a Jloat, enlarging gradually and 

 periodically until the animal has arrived at full maturity.* 



A fifth consideration is had of mechanical advantage, in 

 disposing the Distance at which these successive transverse 

 Plates are set from one another. (See PI. 31. Fig. 1. and 

 PI. 32, Fig, 1, 2.) Had these distances increased in the 

 same proportion as the area of the air-chambers, the ex- 

 ternal shell would have been without due support beneath 

 those sides of the largest chambers, where the pressure is 

 greatest : for this a remedy is provided in the simple con- 

 trivance of placing the transverse plates proportionally 

 nearer to one another, as the chambers, from becoming 

 larger, require an increased degree of support. 



Sixthly, The last contrivance, which I shall here notice, 

 is that which regulates the ascent and descent of the anima] 

 by the mechanism of the Siphuncle. The use of this organ 

 has never yet been satisfactorily ma'de out ; even Mr. Owen's 

 most important Memoir leaves its manner of operation un- 

 certain ; but the appearances which it occasionally presents 

 in a fossil state, (see PI. 32, Fig, 2, 3.,| and PI. 33,) supplv 

 evidence, which taken in conjunction with Mr. Owen's re- 



* In a young Nautilus Pompilius in the collection of Mr. Brodcrip, there 

 are only seventeen Sepia. Dr. Hook says that he has found in some shells 

 as many as forty. A cast, expressing the form of a single air-chamber, of 

 the Nautilus Hcxagonus is represented in Pi. 42, Fig. 1. 



t PI. 32, Fig. 2, represents a fractured portion of the interior of a 

 Nautilus Hexagonus, having the transverse plates (c. c'.) encrusted with 

 calcareous spar ; the Siphuncle a!=o is similarly encrusted, and distended 

 in a manner which illustrates the action of this organ. (PI. 32, Fig. 2, a, 

 a'. a2. a3, d. e. f, and Fig. 3, d. e. f.) The fracture at Fig. 2, b. shows 

 the diameter of the siphuncle, where it passes through a transverse plate, 

 to be iiiuch smaller than it is midway between these Plates (at d. e. f.) 

 The transverse sections at Fig, 2, a. and b , and the longitudinal sections 

 at Fig. 2, d. e. f. and Fig. 3, d. e. f., show that the interior of the siphuncle 

 is filled with stone, of the same nature with the stratum in which the 

 ehcU was lodged. These earthy materials, having entered the orifice of 

 the pipe at a in a soft and plastic state, have formed a cast which shows 

 the interior of this pipe, when distended, to have resembled a string of 



