THE LOWER LIAS OF KEYNSHAM. 
17 
the Sodbiiry area to the norths the sequence is almost 
identical, though the lithological characters are so distinct. 
A thick lower series of Echinid shales is succeeded by thick 
shales in which Am. angulatus is associated with countless 
specimens of Ostrea irregttlaris^ and these beds are capped 
by thin limestones in which Rhynchonella calcicosta occurs, 
though very sparingly. 
In the Eadstock area to the south, no specimen of Am. 
angulatus has ever been recorded, but 0. irregularis is 
common, and the presence of Lima hettangiensis seems to 
point to a considerable development of the Angulatus zone. 
Zone of Am. arietes ( = zones of Bucklandi and Turneri). 
Since, in the Keynsham district. Am. semicostatus., Am. 
turneri, and Am. planicosta occur in the same bed, whilst 
Am. semicostatus occurs lower down with typical speci- 
mens of Am. hisulcatus, we have been compelled to group 
the whole series together under the comprehensive zonal 
designation of Am. arietes. (By the group name. Am. 
arietes, we understand ammonites with a widely open 
umbilicus, a rim provided with a strong central keel flanked 
by deep furrows on either side, and strong, similar, simple 
ribs, running continuously right across the sides of the 
whorls.) This group name' excellently covers the four 
ammonites usually employed as zonal indices, viz.. Am. 
bucklandi, Am. semicostatus. Am. turneri, and Am. 
obtusus. 
This broad division of zones has a very great practical 
advantage in that it requires no expert knowledge of am- 
monites to distinguish between members of the Psilonotus. 
Angulatus, Arietes and Oxynotus groups. 
In order to express more definitely the ranges of species, 
we shall adopt two sub-zones for which we claim, at least, 
a local importance. 
c 
