AMMONITES WERE EXTERNAL SHELLS. 037 
consequently of design, a few of which I shall 
endeavour briefly to point out. 
External Shell. 
The use and place of the shells of Ammonites 
has much perplexed geologists and conchologists. 
Cuvier and Lamarck, guided by the analogies 
afforded by the Spirula, supposed them to be 
internal shells.* There is, however, good reason 
to believe that they were entirely external, and 
that the position of the body of the animal within 
these shells v/as analogous to that of the inha- 
bitant of the Nautilus Pompilius. (See PI. 31, 
Fig. 1). 
* The smallness of the outer chamber, or place of lodgment 
for the animal, is advanced by Cuvier in favour of his opinion 
that Ammonites, like the Spirula, were internal shells. This 
reason is probably founded on observations made upon im- 
perfect specimens. The outer chamber of Ammonites is very 
seldom preserved in a perfect state, but when this happens, it 
is found to bear at least as large a proportion to the cham- 
bered part of the shell, as the outer cell of the N. PompiHus 
bears to the chambered interior of that shell. It often occupies 
more than half, (see PI. 36. a. b. c. d.) and, in some cases, 
the whole circumference of the outer whorl. This open chamber 
is not thin and feeble, like the long anterior chamber of the 
Spirula, which is placed within the body of the animal producing 
this shell ; but is nearly of equal thickness with the sides of the 
close chambers of the Ammonite. 
Moreover, the margin of the mature Ammonite is in some species 
reflected in a kind of scroll, like the thickened margin of the shell 
of the garden snail, giving to this part a strength which would 
apparently be needless to an internal shell. (See PI. 37. Fig. 3. d.) 
The presence of spines also in certain species, (as in A. 
GEOL. Z 
