EXTERNAL SHELL. 19 



nearest the surface being tlie largest. The internal or nacreous 

 la^'er is also distinctly cellular. 



The bod 3^ of the animal of Nautilus, which is api)lie(l to and 

 forms the septum, is of similar smooth and round(Hl form, and 

 we may reason from analogy tliat the arborescent lobes of the 

 septum in tlie fossil Ammonites, were likewise moulded upon a 

 similarly formed body. If we seek a reason fortius complication 

 of structure in Ammonites, we shall find a possible explanation 

 in the connecting siphon which, piercing the centres of the septa 

 in Nautilus, enables the animal thereby to strengthen its hold 

 upon its shell ; but piercing laterally in Ammonites, gives only 

 an excentric attachment. In the latter case, the lobes and rami- 

 fications, applied to the corresponding parts of the animal, would 

 tighten its attachment, and partially remedy the want of the 

 central support. 



The outlines of tlie septa are termed ^nlure^, and when these 

 are folded, as in Ammonites, etc., the elevations are called 

 saddles, and the depressions, lobes. This frilling or folding is 

 found principally at the junction of the septum with the shell- 

 wall, it being nearly flat in the middle, even when its sutural 

 complication is extreme. These parts are subdivided thus : A 

 single dorsal lobe surrounds the siphon, and occupies the medial 

 region of the back, and the additional two lobes on either side of 

 it are named the superior lateral lobes and inferior lateral lobes, 

 whilst other lobes, whatever their numl)er, are called auxiliary 

 lobes ; finall}^, opposed to the dorsal is the ventral lobe. The 

 saddles bear a corresponding nomenclature, that between the 

 dorsal lobe and the superior lateral, being termed tlie dorsal 

 saddle, the next the lateral saddle, etc. 



All multilocular eephalopods have the chambers of their shells 

 connected by a siphon, which traverses the septa, and receives 

 from the latter a series of short investing tubes. 



The siphon pierces the middle of the septa in Nautilus, in 

 Orthoceratites, etc., whilst it is near the internal edge in Aganides, 

 and the dorsal edge in Goniatites, Ammonites, etc. 



It has been supposed by Buckland, that the siphon, which 

 communicates with a large sack, discovered by Owen, in the bod}^ 

 of Nautilus Pompilius, may constitute a hydraulic apparatus by 

 which the animal maj^ vary the weight of its shell at will by 



