356 
ELEMENTS OE GEOLOGY. 
Strata older than the Trias; but, so far as we yet know, they 
did not survive the Liassic epoch. 
Allusion has already been made, p. 354, to numerous zones 
in the Lias having each their peculiar Ammonites. Two of 
these occur near the base of the Lower Lias, having a united 
thickness, varying from 40 to 80 feet. The upper of these is 
Fig. 368. 
Ammonites Bucklandi, Sow. Ammonites hi- 
sulcatus, Brug. One-eighth diameter of 
original. 
a. Side view. b. Front view, showing mouth 
and bisulcated keel. Characteristic of the 
lower part of the Lias of England and the 
Continent. 
Fig. 369. 
A. planorbis, Sow. One-half 
diameter of original. From 
the base of the Lower Lias 
of England and the Con¬ 
tinent. 
characterized by Ammonites BucMandi^ and the lower by 
Ammonites planorbis (see Figs. 368,369).^ Sometimes, how- 
Nautilus truncatiis, Sow. Ammonites bifrons, Brug. A. Walcotii^ Sow. 
Lias. Upper Lias shales. 
ever, there is a third intermediate zone, that of Ammonites 
angulatus^ which is the equivalent of the zone called the 
infra-lias on the Continent, the species of which are for the 
* Quart. Journ., vol. xvi., p. 376. 
Fig. 370. 
