THE LOWER LIAS OF KEYNSHAM. 
45 
rim square cut, strongly bisulcate and tricarinate ; ribs 
about forty-five, close set, strong and narrow, with a 
distinct tendency to form points near the inner margin, 
as well as near the outer edge ; from the outer of these 
points the ribs bend forward towards the rim, and from the 
inner they bend forward towards the innermost edge of the 
whorl. A characteristic feature is a shallow depression on 
the side of the whorl between the outer points of the ribs 
and the edge of the rim, in which the ribs die out. 
Ammonites of the Semicostate group {Arnioceras). 
Am. semicostatus., Y and B. 
The most important character of this group is the absence 
of ribs from the earlier whorls. The ribs are simple, tall 
and narrow, and curve or bend forward near the edge, where 
they attain their greatest height. The rim is bisulcate with 
a prominent keel. The whorls scarcely embrace (so that 
fragments of whorls are extremely common). 
There are two well-marked varieties : 
(1) Am. obliquecostatus^ Ziet. The whorl section is as 
wide as tall ; the ribs sweep forward near- the edge with 
continuous curvature ; they strike the inner margin some- 
what obliquely. The keel is very tall, so as to be con- 
spicuous in a side view of the test. 
This form approaches to the bisulcate group, but differs 
radically in its semicostate character. 
(2) Am. geometricus., 0pp. Whorl section rectangular, 
narrower than tall ; ribs run straight across the sides and 
reach their greatest height near the edge, where they bend 
abruptly forward toward the rim. 
Am. turneri^ Sow. is distinguished from Am. semicostatus^ 
with which it occurs, by the absence of smooth inner 
whorls and by the broad, forward sweep of the ribs as 
they approach the rim. 
