260 TRANSVERSE PLATES AND AIR-CHAMBERS. 



Similar tubercles are introduced with the same advantage 

 of adding Strength as well as Beauty in many other cognate 

 genera of chambered shells. (PI. 44, Fig. 9. 10. 14. 15.) 



In all these cases, we recognise the exercise of Discretion 

 and Economy in the midst of Abundance ; distributing in- 

 ternal supports but sparingly, to parts which, from their ex- 

 ternal form, were already strong, and dispensing them abun- 

 dantly beneath those parts only, which without them, would 

 have been weak. 



We find an infinity of variations in the form and sculpture 

 of the external shell, and a not less beautiful variety in the 

 methods of internal fortification, all adapted with archi- 

 tectural advantage, to produce a combination of Ornament 

 with Utility. The ribs also are variously multiplied, as the 

 increasing space demands increased support ; and are vari- 

 ously adorned and armed with domes and bosses, wherever 

 there is need of more than ordinary strength. 



Transverse Plates, and Mr Chambers. 



The uses of the internal air-chambers will best be under- 

 stood by reference to our figures. PL 36 represents a lon- 

 gitudinal section of an Ammonite bisecting the transverse 

 plates in the central line where their curvature is most sim- 

 ple. On each side of this line, the curvature of these plates 

 become more complicated, until, at their termination in the 

 external shell, they assume a beautifully sinuous, or foliated 

 arrangement, resembling the edges of a parsley leaf, (Pi. 

 38,) the uses of which, in resisting pressure, I shall farther 

 illustrate by the aid of graphic representations. 



portion of this species. Between these three keels, or ridg^es, are two de- 

 pressions or dorsal furrows, and as these furrows form the weakest portion 

 of the shell, a compensation is provided by conducting- beneath them tiic 

 denticulated edges of the transverse plates, so that tliej' present long lines 

 of resistance to external pressure. 



