1G6 



The lobes and saddles vaiy with age in different species ; not only do 

 their ramifications increase in complexity during the whole life of the 

 animal, but their number augments with growth. According to voN 

 BucH, all Ammonites have only six lobes, the dorsal, ventral, and two 

 lateral. In some species this number continues the same through life, 

 whilst in others, one, two, or more auxiliary lobes are added to the laterals 

 on each side ; and their number and development increase with the age of 

 the individual. Notwithstanding this change in the form and number 

 of the lobes of Ammonites with age, and which detracts much from their 

 diagnostic value in the detennination of the species, still they constitute 

 an irnportant feature in their history, and their special foi-m and arrange- 

 ment should always be carefully noted whenever they are seen, or can 

 be exposed by the removal of a portion of the wall of the shell. 



The late M. A. d'OREiGNY stated that after having made reasonable 

 deductions for the varieties due to age, sex, and pathological conditions, 

 he estimated the number of Ammonites to amount to 530 species ; so 

 that it is indispensable to classify them into groups in order to facilitate 

 their determination. With this view, Sowers y, in his Systematical 

 Index of tlie Mineral Conchology, vol. vi., p. 249, divided the Ammonites 

 into three sections — 



1st Margin rounded, 



2nd Margin flattened, 



3rd Margin keeled, 



the latter subdivided into (a) keel entire, and (b) keel crenated. 



Von Buch, in 1829, divided Ammonites into twelve groups, the 

 characters of which were founded upon the external fonn and arrangement 

 of the septa, which he had then just described for the first time. These 

 groups are — 



EXAMPLE DISTRIBUTION 



A. Bucklandi, Sow. Lower Lias 



A. serpentinus, Schloth. Up. Lias and Low. Oolites 



A. margaritatus Mont. Middle Lias and Oolites 



A. Gapricornus,^cla\ot\\. Middle and Upper Lias 



A. triplicatus, Sow. Upper Lias and Oolites 



A. armatiLS, Sow. Lias 



A. Henleyi, Sow. Lias and Oolites 



A. Herveyi, Sow. Oolites 



A. Ehotomagetisis, d'Orh. Cretaceous 



A. spleiidens, Sow. Oolitic and cretaceous 



A. Duncani, Sow. Oolitic 



A. radiatus, Brug. Lower Cretaceous 



