16 CEPHALOPODA. AMMONITES. 
this conclusion, from the smallness of the outer chambers. But this theory 
seems to have been founded on observations made on imperfect specimens. 
When the outer chamber is obtained in its perfect condition, it is found to be 
as large in proportion as the outer cell of Nautilus Pompilius, (plate I. fig. 3,) 
is-to the chambered convolutions of that shell. It will be seen by our section 
of Ammonites ebtusus, plate I. fig. 4, that it often occupies nearly half the 
circumference of the outer volution, from @ to 0b; and in other instances it 
embraces nearly the entire outer volution, as represented from to7z. Unlike 
the thin fragile exterior chamber in Spiruda, that of the Ammonites is nearly 
as thick and strong as the close chambered portion. 
Besides what we have above shewn, the spinous exterior of many of them, 
suchas A. Gowerianus, armatus, &c. presents a striking argument against their 
being internal shells; as those processes, which seem to be destined as a pro- 
tection to the animals against their enemies, would not only be useless, but 
irritating to the soft integuments of a mollusk. 
Ammonites giganteus. Plate 11, fig. 17. Found in sandy limestone, in 
Chicks-grove quarry near Hindon, Wiltshire; it has also been met with at 
Purbeck Isle, Dorsetshire, Marlborough downs, and in the Chalk, near Mar- 
gate. It is one of the largest of the genus. There is a specimen in the Jardin 
des Plantes, Paris, which measures four feet in diameter. We have given a 
representation of the winding partitions which intervene between the air 
chambers of A. giganteus, plate I. fig. 1. 
A. heterophyillas, plate I. fig. 17, from the Liasat Whitby. It is a longi- 
tudinal view of the fossil, the transverse plates which approximate so closely 
on the sides of the shell, where it is flat and weak, are seen to be distant from 
each other along the dorsal region, which is strong from its convex form. 
The small circular black spot marked a, is the siphuncle; 06 the dorsal 
lobe; c the dorsal saddle; @ the superior lateral lobe; ¢ the lateral saddle ; 
Jf the inferior lateral lobe; g the ventral saddle; % ventral lobe; i 7% 
axillary lobes. The situation in which the siphuncle passes through the 
partitions, is represented in plate I. fig. 19, c. 
It has been demonstrated by De La Beche, from specimens obtained in the 
Lias at Lyme Regis, Dorsetshire, that the entire body of the animal was con- 
tained within the outer chamber; and that these mollusks must have been 
suddenly annihilated, and entombed in the earthy deposit of which the Lias is 
composed, before their bodies had been affected by decomposition, or had 
been devoured by the crustaceous carnivorous animals, which abounded in the 
former ocean. 
There is considerable difficulty in the discrimination of species of the genus 
Ammonites, as many of them differ very materially in their progress from 
their infant to their adult condition. 
In the Goniatites subleves,* there is a great dissimilarity of form between 
the young and old shells. In the infant state, when the shell is a quarter of an 
inch in length, it is only half that breadth, and is provided with distinctly 
marked transverse small ribs; as the shell advances in size, these become more 
rounded and obtuse, and are alternately long and short, and some of them 
furcated, When it has reached the length of from two and a half to three 
inches, these ribs become almost obsolete, and when the shell has acquired its 
full growth of four or five inches, they can be traced with the utmost difficulty, 
and sometimes are invisible ; and the shell has become almost spherical. 
In carefully tracing its progress, it will be observed, that the chambers 
* Brown’s Illustrations Foss. Conch. plate XVI. fig. 6. 
