42 
THE LOWER LIAS OF KEYNSHAM. 
the tall, close-set, erect ribs ‘which sweep gently forward and 
end abruptly on each side of the rim, so as to leave a well- 
defined, smooth parting along the middle of the rim. The 
angle at which the ribs on one side would meet those on the 
other, if produced across the smooth parting, is always 
broadly obtuse and, in the adult, very nearly 180°. In 
the young form, the ribs are sharp and strongly developed 
over the whole of the whorl side, attaining their greatest 
height on the edge of the rim. It is a direct consequence of 
this fact that, when the ribs tend to disappear in old age, 
they are still conspicuous as strong crenulations on the 
edges, whilst the sides become quite smooth. This species is 
subject to only slight variation, which takes place chiefly in 
the greater or less alternation of the ribs. The broad angle 
between the ribs on either side completely separates this 
species from any mutation of Am. angidatus. 
Ammonites of the Bisulcate group {Coroniceras). 
This group is easily identified by the broad bisulcate and 
tricarinate rim, the wide umbilicus, the strong, simple ribs, 
running completely across the sides, and the large size to 
which the adult form attains. The ribs are, as a rule, simply 
and gently curved, and have a tendency to develop knobs 
near the rim edge ; the ribs are never conspicuously absent 
from the early whorls, as in the following group : — 
The suture line is essentially the same throughout all the 
mutations of the group and, being also usually conspicuous, 
it forms an important determinative character. Once care- 
fully examined, there can be no difficulty in recognizing the 
peculiarity of the type, but since a minutely technical ex- 
position of suture lines is seldom illuminating, it is sufiicient 
to draw attention to the deep, tridigitate rim lobe, and to the 
broader, but shallower, tridigitate principal lobe. 
The group can be readily split up into two sections, one 
