2-22 
.V 
THE LIAS AMMONITES. 
Von Buch, 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 Haau. 
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. 
Fig. 106.— Side view. Fig. 107 —Ventral view. 
Ceratites nodosus, De Haaii. 
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 rounded 
saddles look forwards. Those parts are more com- 
plicated in Amaltheus margaritatus (fig. 100), and 
still more so in Plii/lloceras lieterophyllum (fig. 102), 
in which the auxiliary lobes are numerous and well 
developed. Eigs. 101, 102 show the disposition 
of the lobes of this beautiful Ammonite from the Upper Lias. 
Secondly shape of the aperture and the structure and development of the 
mouth-border of the shell vary very much in the diff'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 difi'erences do exist, 
and, perhaps, many more will be discovered by continued researches made along the 
same line of investigation. 
In Arieiites, as seen in Arietites rotiformis (fig. 108) and Arietites ohtusus (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 
