MODERN CLASSIFICATION. 



227 



Thirdly. — 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 aifords 

 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 

 horaologue of the Apfychis. 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. 



-Nautilus umbilicatua. 

 Lam. 



Frs. 129. — Vertical section of the shell of Nautilus 

 pompilius, showing the siphuncle and the situa- 

 tion of the animal in the last chamber. 



Fig. 131.— Section of the shell 

 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 sitit 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 M'horl ; 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 Naufihis 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. 



