AMMONITES WERE EXTERNAL SHELLS. 25.5 



seating the inner portion of the shell, (See PI. 36 and 41.) 

 3d. A siphuncle, or pipe, commencing at the bottom of the 

 outer chamber, and thence passing through the entire series 

 of air-chambers to the innermost extremity of the shell, (see 

 PI. 36, d. e. f. g. h. i.) In each of these parts, there are 

 evidences of mechanism, and 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 La- 

 marck, 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 



* The smallncss 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 obser- 

 vations made upon imperfect 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 chambered part of the 

 shell, as the outer cell of the N. Pompilius 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 out whorl. This open cham- 

 ber 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 Ammo- 

 nite. 



Moreover, the margin of tiie 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. Armafus, A. Sovv- 

 erbii,) affords a strong argument against the theory of their having been 

 internal shells. These spines which have an obvious use for protection, if 

 placed externally, would seem to have been useless, and perhaps noxious in 

 an internal position, and are without example in any internal structure with 

 which we are acquainted. 



