THE LOWER LIAS OF KEYNSHAM. 
19 
in number of ribs, but the commonest form is one with 
numerous ribs. In consequence, it seems preferable to select 
a group name as index, rather than the name of a particular 
and rather aberrant mutation which is certainly not the 
commonest variation of the general form. 
Am. bisulcatuSj as first defined by Bruguiere, covers the 
whole group excellently and has been used in the same 
extended sense by d’Orbigny in France and Brauns in 
Germany. 
Sub-zone of Am. semicostatus. 
In this division we include all the beds from the perfect 
shale, containing fish remains, up to the base of the thick 
clay which forms the top of the exposures in which these 
beds occur. 
The semicostate (or arniocerate) ammonite group which 
characterizes these beds is marked out by a strong keel, 
flanked by furrows, and strong, simple ribs ; but, above 
all, by having the earlier whorls conspicuously smooth. 
We further subdivide this sub-zone into a lower and 
upper portion characterized respectively by two varieties of 
Am. semicostatus., viz., the lower by Am. obliquecostatus 
and the upper by Am. geometricus. As already stated, 
many geologists might be inclined to place the lower of 
these subdivisions in the Bisulcatus zone. The reasons for 
such a view are : — 
1. The close lithological similarity of the hard bed 
in this subdivision to the hard beds of the Bisulcatus 
zone and its dissimilarity to the rubbly, discontinuous 
limestone bands in the beds above. The Obliquecostatus 
subdivision usually consists of a thick, perfect shale, suc- 
ceeded by a hard limestone in which the index species is 
extremely abundant. When this thick shale thins out and 
the rubbly limestone bands above degenerate into mere beds 
