VON buch\s theory of ammonites. '353 
resist the pressure of deep water, M. Von Buch 
has suggested a further use of the lobes thus 
formed around the base of the outer chamber, 
as affording points of attachment to the mantle 
of the animal, whereby it was enabled to fix 
itself more steadily within its shell. The ar- 
rangement of these lobes varies in every species 
of Ammonite, and he has proposed to found on 
these variations, the specific character of all the 
shells of this great family.* 
* The most decided distinction between Ammonites and Nau- 
tili, is founded on the place of the siphon. In the Ammonite, 
this org-an is always on the back of the shell, and never so in the 
Nautilus. Many other distinctions emanate from this leading: 
difference ; the animal of the Nautilus having its pipe usually 
fixed near the middle, (See PI. 31, Fig. 1), or towards the ventral 
margin (PI. 32, Fig. 2, and PI. 42, Fig. 1.) of the transverse 
plates, is thereby attached to the bottom of the external chamber, 
which is generally concave, and without any jagged termination, 
or sinuous flexure, of its margin. As the siphon in Ammonites 
is comparatively small, and always placed on the dorsal margin 
(PI. 36, d. and PI. 39, d.), it would have less power than the 
siphuncle of Nautili to keep the mantle in its place at the bottom 
of the shell ; another kind of support was therefore supplied by a 
number of depressions along the margin of the transverse plate, 
forming a series of lobes at the junction of this plate with the 
internal surface of the shell. 
The innermost of these, or ventral lobe, is placed on the inner 
margin of the shell (PI. 39, V.) ; opposite to this, and on the 
external margin, is placed the dorsal lobe (D), embracing the 
siphon and divided by it into two divergent arms. Beneath the 
dorsal lobe are placed the superior lateral lobes (L), one on each 
side of the shell ; and still lower, the inferior lateral lobe (1), 
next above the ventral lobe. 
The separations between these lobes form seats, or saddles,, 
GEOL. A A 
