222 



THE LIAS AMMONITES. 



Von Biicli, d'Orbigny, Quenstedt, and Oppel. I have given a resume (pp. 181, 182) of the 

 descriptions of the lobes and ramifications of the margin of the septa in treating of the 



Fig. 103. — Disposition of the lobes (l, l', 

 a, a) in Ceratites nodosus, De Haan. 



Fig. 104. — Form of the subdivisions of the 

 lobes (l) and saddles (s) in Ammonitidae. 



Fig. 105. — Mould of the disposition 

 of the lobes (l, d, v) of Phylloceras. 



classification of the Ammonitid^, and have shown that the points of the foliations are all 

 directed backwards towards the winding of the spire, and the convexities all look forward 



towards the aperture (as seen in figs. 100, 102) in 

 the ramified foliations of the suture-line ; where 

 tliey are folded the elevations are called saddles 

 and the intervening depressions lobes. In the 

 Ceratites these parts are very simple, as shown in 

 Ceratites nodosus (fig. 106), where the dentated 

 lobes are seen to point backwards and the romided 

 saddles look forwards. Those parts are more com- 

 plicated in Amaltheus margaritatus (fig. 100), and 

 still more so in Phylloceras lieterophyllmn (fig. 102), 

 Fig. 106.— Side view. Fig. 107 —Ventral view, jj^ which the auxiliary lobes are numerous and well 



Ceratites nodosus, De Haan. '' 



developed. Eigs. 101, 102 show the disposition 

 of the lobes of this beautiful Ammonite from the Upper Lias. 



Secondly. — The shape of the aperture and the structure and development of the 

 mouth-border of the shell vary very much in the difi'erent groups, and the special form 

 which the border presents in different Ammonites affords an element of importance in 

 defining the diagnosis of genera. The difficulty which the palaeontologist experi- 

 ences in obtaining Ammonite shells, or moulds of such, in which the aperture with 

 its border is preserved, has long prevented this anatomical character from being 

 used in constructing the diagnosis of the different groups ; at length, however, many 

 specimens have been found, and we have ascertained that important differences do exist, 

 and, perhaps, many more will be discovered by continued researches made along the 

 same line of investigation. 



In Arietites, as seen in Arietites rotiformis (fig. 108) and Arietites obtusus (fig. 109), 

 the sides of the mouth-border are simple, and the ventral portion is more or less pro- 

 duced, as indicated by the forward direction of the ribs in this region, these folds of the 



