186 THE LIAS AMMONITES. 
reasonable. That the fibrous basis of the hood was retained in different degrees in the 
Ammonites is indicated by the simply corneous or chitinous condition of the Ajitychus 
which has been preserved in some examples of " Harpoceras Levisoni, Aegoceras planorbis, 
SoAv., PI. XIV, fig. 3, and many other forms from the Lias, and O^jpelia subradiata. 
Sow., from the Inferior Oolite of which a beautiful specimen with its Aptychus in situ 
is to be seen in the British Museum.^ 
" The conjoined plates of the Aptychus (figs. 47, 48, 49) form a triangular disc, of which 
the base is backward, excavated to receive the involute part of the shell, with the sides of 
the base, like the corresponding lobes of the hood, bent down to cover the laterally 
extended parts of the wider terminal coil of the shell. Even in the contrast between the 
papillose wrinkled outer surface and the smooth inner surface of the ' hood ' of Nautilus 
the resemblance to the ' Aptychus ' of the Ammonite is carried out." 
Leopold von Buch,^ in 1830, introduced an important reform in the grouping of 
Ammonites by showing how the foliations of the septa retained certain fixed forms in the 
species, and considerable differences in each of the twelve groups into which he classified 
these fossils, and which he thus defined : 
I. Arietes.— Upon the sides of the whorls a large number of thick simple ribs 
or radii are developed, which all bend forward near the back. The siphuncle projects 
outwards from a channel which extends on each side of it, and this bisulcation of the 
siphonal area separates the ribs on the sides from each other (fig. 52). The lobes of the 
division wall of the chambers have the following form : — The siphonal is almost as deep 
the size of the dwelling 
chamber. 
as it is broad. The point of adhesion to the sheath of the siphuncle is exactly in the 
middle of its depth. The upjjer lateral does not extend to half its depth, and is sometimes 
^ Described by Dr. S. P. Woodward in a short memoir on " An Ammonite from the Inferior OoHte 
with its operculum in situ," ' Geologist,' vol. iii, p. 328, 1860. 
* ' Ueber die Ammoniten in den alteren Gebirgs-Schicbten,' p. 135; Akademie der ^Yissenschaften 
zu Berlin, ausdem Jahre 1830. 
