VON buck's theory of ammonites. 267 



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 arrangement of these lobes varies in every species of 

 Ammonite, and he has proposed to found on these varia- 

 tions, the specific character of all the shells of this great 

 family.* 



* The most decided distinction between Ammonites and Nautili, is founded 

 on the place of tlie siphon. In the Ammonite, this organ is always on the 

 hack of the shell, und 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 tlie 

 ■ventral margin (PI. 32, Fig. 2, and PI. 42. P"ig. 1.) of the transverse plates, 

 is thereby attached to the bottom of the external chamber, whicii is gene- 

 rally 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 tiie margin of the transverse plate, forming a 

 series of lobes at the junction of this plate witii 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 

 tlie dorsal lobe (D,) embracing tlie siphon and divided by it into two diver- 

 gent arms. Heneath 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, upon which 

 the mantle of the animal rested, at the bottom of the outer chamber; these 

 saddles are distinguished in the same manner as the lobes — that between 

 the dorsal and superior lateral lobe, forming the dorsal saddle (S. d.,) tiiat 

 between the superior and inferior lateral lobes, forming the lateral saddle 

 (S. L.,) and that between the inferior lateral and ventral lobe, the ventral 

 saddle (S. V.) This general disposition, variously modified, pervades all 

 forms of Ammonites; but when, as in PI. 39, the turn of the shell increases 

 rapidly in width, so that the last whorl nearly, or enlireljs covers the pre- 

 ceding whorls, the additional part is furnished with small auxiliary lobes, 

 varying with the growth of the Ammonite to the number of tliree, four, or 

 five pairs, (Pi. 39, a. 1, a. 2, a. 3, a. 4, a. 5.) 



