34 THE SHELL. 



characteristic of the latter, being here quite indistinguishable. 

 The shell of Nautilus is the onl^^ one in which the presence of 

 two layers is obvious, from their difference of texture. A thin 

 section of the external layer of the shell of Nautilus Pompilius, 

 taken parallel to the surface, shows that it is made up of an 

 aggregation of cells of various sizes, those strata which are 

 nearest the surface being the largest. The internal or nacreous 

 layer is also distinctly celkilar. 



The body of the animal of Nautilus, which is applied to and 

 forms the septum, is of similar smooth and rounded form, and 

 we may reason from analogy that the arborescent lobes of the 

 septum in the fossil Ammonites, were likewise moulded upon a 

 similarly formed surface. If we seek a use for this 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 excentrie attachment. In the latter case, the lobes and 

 ramifications, applied to the corresponding parts of the animal, 

 would tighten its attachment, and partially remedy the want of 

 the central support. 



The outlines of the septa are termed sutures, and when these 

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

 saddles, and the depressions, lobes (xxxv, 12 ; ii, 28, 29). This 

 frilling or folding is found principally at the junction of the 

 septum with the shell-wall, the former being nearly fiat in the 

 middle, even when its sutural complication is extreme. These 

 parts are subdivided thus : A single siphonal or ventral 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 number, are called auxiliary lobes; finallj^, 

 opposed to the siphonal is the antisiphonal lobe. The saddles 

 bear a corresponding nomenclature, that between the ventral 

 lobe and the superior lateral, being termed the ventral saddle, 

 the next the lateral saddle, etc. 



All multilocular cephalopods 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 (ii. 23), 

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

 Clymenia (ii, 24), and the outer edge in Goniatites, Ammonites, 

 etc. (ii, 25). 



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

 communicates with a large sack, discovered by Owen, in the body 

 of Nautilus Fompilius, may constitute a h3'draulic apparatus by 

 which the animal may vary the weight of its shell at will by 



