14 CEPHALOPODA. AMMONITES. 
interior wall articulated by a sinuous suture; septa transverse, 
lobed at their circumference, and perforated by a marginal 
siphuncle. 
Orbulites striatus. Plate II. fig. 5. Locality unknown. 
Known only in a fossil state. 
Sowerby thinks that this shell should not rank as a distinct genus, inasmuch 
as he considers the circumstance of the last volution covering the others, as an 
insufficient distinctive character. In this, however, we differ from him. 
Genus X.— AMMONITES. — Lamarck. 
Generic Character. — Shell discoidal, multilocular; vo.u- 
tions contiguous, all of them visible to a greater or less 
extent, gradually and spirally coiled around the first or 
central one, so as to exhibit each volution increasing pro- 
gressively in size outwardly, and more or less compressed ; 
inner partitions articulated by sinuous sutures ; septa trans- 
verse, lobed at the circumference, and imperforated at the 
disk, but perforated by a single tube or siphuncle, situate 
near the outer margin or back of the shell; aperture in 
most species thickened, expanded outwards in some, and in 
others, contracted. 
Ammonites giganteus. Plate II. fig. 17. 
The situation of the siphuncle is a very decided character in this genus. It 
is invariably placed in the aperture, close to the dorsal margin or ambit, pene- 
trating the transverse partitions; as exhibited in plate I. fig. 4. It is repre- 
sented in black, and marked by the letters c, d, e, f, g, h. It is conducted 
through the septa by a ring projecting outwards, and may be traced pene- 
trating through the whole transverse partitions, or plates of the figure above 
referred to. In most of the carinated species, the siphuncle is placed in the 
keel; while in the genus Nautilus, the siphuncle is either in the centre of the 
convolutions, or towards their inner sides. The body of the animal has occu- 
pied that portion of the shell from a to b in A. Obtusus, plate I. fig. 4. 
The delicate dorsal siphuncle of the Ammonite, is not sufficient to prevent 
concussion of the animal within its partitions, Other means of attachment are 
accordingly necessary, and the following provision is found. Below each par- 
tition, there lie six lobes, symmetrically arranged around the circumference of 
the shell. The first or ventral lobe, is usually the most considerable, and 
rests upon the back of the convolution which precedes it. On the opposite 
side, the dorsal lobe advances itself towards the bottom to embrace the 
siphuncle, and it is thereby divided into two cones, which are more or less 
separated from one another. At one-third the height of the aperture from 
the back, the superior lateral lobes are placed on each side, and lower down 
the inferior lateral lobes, are similarly arranged—the latter being a little more 
elevated than the ventral lobe. The separation of these lobes form the sell, 
or seats, so termed because the animal rests upon them, and their distinctive 
tnames are derived from those of the lobes 
Some of the Ammonites are plain, but most of them are variously sculp- 
ured, and many are annulated like a ram’s-horn, from which circumstance, in 
all probability, they have their common name, Cornua Ammonis, or Am- 
