ADDITION OF CHAMBERS. 245 



much avail as ribs to increase the aggregate amount of 

 strength. (See PI. 32, Fig 1. a. to b.) 



Thirdly, the arch is rendered still stronger by the disposition 

 of the edges of the internal Transverse -plates, nearly at right 

 angles to the sides of the external shell, (See PI. 32, Fig. 1, 

 b. to c.) The course of the edges of these transverse plates 

 beneath the ribs of the outer shell is so directed, that they 

 act as cross braces, or spanners, to fortify the sides of the 

 shell against the inward pressure of deep water. This con- 

 trivance is analogous to that adopted in fortifying a ship for 

 voyages in the Arctic Seas, against the pressure of ice-bergs, 

 by the introduction of an extraordinary number of trans- 

 verse beams and bulk heads.* 



We may next notice a fourth contrivance by which the 

 apparatus that gives the shell its power of floating, is pro- 

 gi'essively enlarged in due proportion to the increasing 

 weight and bulk of the body of the animal, and of the ex- 

 ternal chamber in which it resides ; this is eflected by suc- 

 cessive additions of new transverse Plates across the bot- 

 tom of the outer chamber, thus converting into a'lr-ch ambers 

 that part of the shell, which had become too small to hold the 

 body. This operation, repeated at intervals in due propor- 



* The disposition of the curvatures of the transverse ribs, or lines of 

 growth, in a different direction from the curvatures of the internal transverse 

 plates, affords an example of farther contrivance for producing strength in 

 the shells both of recent and fossil Nautili. As the internal transverse plates 

 are convex inwards, (see PI. 32, Fig-. 1, b. to c.) whilst the ribs of the outer 

 shell are in the greater part of their course convex outwards, these ribs in- 

 tersect the curved edges of the transverse plates at many points, and thus 

 divide tiieni into a series of curvilinear parallelograms ; the two shorter 

 sides of each parallelogram being formed by the edges of transverse plates, 

 whilst its two longer sides are formed by segments of the external ribs. 

 The same principle of construction here represented in our plate of Nautilus 

 hexagonus, extends to other species of the family of Nautilus, in many of 

 which the ribs are more minute; it is also applied in other families of fossil 

 ehambered shells; e. g. the Ammonites, PI, 35, and PI. 38. Scaphitcs, PI,. 

 44, Fig. 15. Ilamites, Pi. 44, Fig. 8_I3. Turrilites, PI. 44, Fig., 14, and. 

 Baculites, PI. 44, Fig. 5. 



21* 



