THE LOWER LIAS OF KEYNSHAM. 
‘27 
much higher horizon, namely in the Jame.soni beds, though a 
mutation, Rli. plicatissima^ which also occurs at Keynsham, 
is found as low down as the Arietes zone). 
Large Limas occur sparingly at the base, but do not range 
to the top of the series ; 0. irregularis also becomes less 
common, and is replaced by Gryphea arcuata, which becomes 
more and more abundant from bed to bed, until it reaches its 
maximum in the top beds of this series, and in the lowest 
beds of the next. 
As has been already noticed, the ammonite zones overlap 
at the top of this series, for the top block contains bisulcate 
ammonites in considerable number, whilst a rare specimen of 
Am. angulatus extends into the beds immediately above. 
Fragments of Am. angulatus of all ages are not uncommon 
throughout the lower beds of the series. Am. cJiarmassei 
enters in the top block, but reaches its maximum in the 
shale immediately above, and is more characteristic of the 
next zone {Arietes zone). A very few specimens of Bel. 
acutus have been observed in the top block, but this fossil 
does not become abundant until the very top of thQ Arietes 
zone. 
The Arietes beds, which succeed the Calcicosta series, 
reach a total thickness of twelve feet ; the lower six feet is 
made up of massive limestones, separated by subsidiary clay 
bands (zone of Am. bisidcatus), whilst the upper six feet com- 
prises thick shales with subsidiary limestone bands (zone of 
Am. semicostatus). 
{a) Zone of Am. hisulcatus. 
At the base, Am. charmassei and Gryphea arcuata each 
reach a maximum, Pentacrinus fragments are extremely 
common, Rh. calcicosta in its typical form dies out, and the 
most characteristic bisulcate ammonite is the conyheari 
mutation. 
Throughout the main portion, mutations of bisulcate 
