MODERN CLASSIFICATION. 
227 
TUrdhj. — If the structure of the mouth-border shows that Ammonites varied much 
in the anatomy of their head, mantle, and arms, the length of the body-chamber affords 
another character from which important deductions may be made. The capacity of this 
chamber was doubtless proportioned to the size of the contained animal, as seen in the 
Nautilus pompilius (fig. 129) of our seas, where the Mollusc is represented occupying the 
entire body-chamber, the outer margin corresponding to the ventral surface, and the 
columellar side to the dorsal surface of the animal. We have here likewise to note 
the position of the tentacles and the place of the hood, which may probably be the 
homologue of the AjjtycMs. The septa are likewise seen to be concave towards the 
aperture, and convex towards the spire of the shell, and they all are perforated at 
the centre by the siphuncular tube, which proceeds from the posterior side of the 
Cephalopod to traverse the septa up to the last air-chamber. 
Fig. 130. — Nautilus urnbilicatus, pompilius, showing ilie siphuncle and the situa- Fig. 131. — Section of the shell 
Lain. tion of the animal in the last charaher. of Nautilus pompilius, Linn. 
In fig. 131 we have a vertical section of the Nautilus shell (fig. 130). The body 
in the section is seen to extend to one half of a whorl, and from the inner side of 
each septum we observe a short funnel-shaped process projecting backwards, which 
affords attachment to the membranous siphuncle seen fully in situ in fig. 129. The 
length and capacity of the body-chamber vary considerably in the different genera, and 
are found to have proportionate dimensions to that of the outer whorl ; this appears 
to be constant in several of the groups on which careful observations have been made ; the 
difficulty experienced in obtaining specimens in which the body-chamber is preserved 
is, however, a sufficient reason why more has not hitherto been done in this line of 
inquiry. 
In Arietites (fig. 132) it will be observed that the septa are convex towards the 
aperture and concave towards the spire, whilst in Nautilus we have seen that the 
opposite condition prevails. The body-chamber in Arietites is likewise very long, 
extending from one whorl to one whorl and a half in length. 
