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 Apti/chus 

 which has been preserved in some examples of " Harpoceras Levisoni, Aegoceras planorbis. 

 Sow., PI. XIV, fig. 3, and many other forms from the Lias, and O^Jjjelia mhradiata. 

 Sow., from the Inferior Oolite of which a beautiful specimen with its Aptychis in situ 

 is to be seen in the British Museum.^ 



" The conjoined plates of the Apiyclms (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 dififerences 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 sijjhonal is almost as deep 



Fig. 50. — Arietites rotiformis, 

 Sow. Side view. 



Fig. 51.—/ 



Sow. Side vievi'. 



Fig. 53. — Section of Arietites 

 o//tttsus. Sow. Sliowing 

 the size of the dwelling 

 cliamber. 



as it is broad. The point of adhesion to the sheath of the siphuncle is exactly in the 

 middle of its depth. The upper lateral does not extend to half its depth, and is sometimes 



^ Described by Dr. S. P. Woodward ia a sliort memoir on " An Ammonite from the Inferior Oolite 

 with its operculum in situ," 'Geologist,' vol. iii, p. 328, 1860. 



3 ' Ueber die Ammoniten in den alteren Gebirgs-Schichten,' p. 135 ; Akademie der Wissenscbaften 

 zu Berlin, ausdem Jahre 1830. 



