AMMONITES— ZOOLOGICAL, ETC. 



103 



Natural varieties of the Septa.— 1 have not observed any- 

 natural varieties in the Septa. All taken at the same age 

 afford precisely the same general characters, and do not vary- 

 but within very narrow limits, and simply in the ultimate 

 details of the small foliations of the lobes and saddles. I 

 shall nevertheless adduce an instance which determines some 

 slight variations, not in the general form or details, but in the 

 elongation or shortening of these parts, without producing 

 any other modification. This instance generally takes place 

 in the last Septa, which an Ammonite has formed before 

 its death, and appears to be caused by disease or old age. It 

 takes place in the Septa which approach nearest each 

 other. These, instead of maintaining the regular increasing 

 distance, approach suddenly, and in a considerable degree in 

 the three or four last ones ; and then, all the ramifications of 

 the lobes and saddles not being able to attain their usual 

 extension, it folloAvs that these parts become considerably 

 shortened, still preserving the same width. We may say, 

 as a general rule, that the larger the chambers in a species, 

 the more the foliations of the lobes are elongated. I look 

 upon the increasing separation of the Septa, as the sign of a 

 perfect state of health in the Ammonite, whilst their ap- 

 proximation clearly indicates an unhealthy condition. I 

 have not remarked any difference arising from sex. 



Varieties of the Septa from age, — Though the general 

 form of the Septa is invariable in the six primitive lobes, (the 

 dorsal, ventral, two superior lateral, and two inferior lateral 

 lobes,) it is by no means so, as regards the complication of 

 their ornaments, and the number of the auxiliary lobes, 

 which all vary according to age, but within very different 

 limits from those of the external appendages. 



1. — An Ammonite taken in the embryo state, for 

 example, whatever may be the ultimate number of the 

 auxiliary lobes, is always furnished with the six primitive 



