'28 THE LOWEE LIAS OF KEYNSHAM. 
ammonites abound and include the types usually named 
bucklandi, miiltico status and hisulcatus. In our opinion 
these names only indicate a few links, arbitrarily chosen, in a 
complete chain of mutation. Besides the typical Am. hisulcatus^ 
nearly allied coronicerates, typified by ?’of^ybrm^s, are far 
from uncommon, especially towards the base {Am. rotiformis 
itself and the striking mutation which we identify with Am. 
coronaries^ Quenst. are the two commoner forms). Rhynclio- 
nella triplicata (a member of the Rli. variabilis group) 
succeeds Rh. calcicosta and is quite characteristic of all the 
beds which contain large bisulcate ammonites. 
In the top beds, though large bisulcate ammonites clearly 
indicate the zone to which it belongs, the occurrence of large 
nautili, Avicula inequivalvis^ and the semicostate Am. 
ohliquecostatus link the bed with those just above 
(b) Zone of Am. semicostatus. 
This is characterized throughout by the abundance of semi- 
costate ammonites {Arnioce7‘as) of this type. Am. oblique- 
costatus ^ which is more nearly allied to the bisulcate group, 
is typical of the lower beds, whilst the more characteristic 
mutation, Am. geometricus^ crowds the shales or thin lime- 
stones of the upper beds. 
Rhynchonella semicostati {v.i.), which we consider closely 
allied to the Badstock type of Rh. lineata^ is extremely 
abundant at more than one horizon, though usually crushed. 
Avicula inequivalvis and Anomia pellucida occur in 
thousands in the shales at the base, and the former is equally 
common in the upper portion, associated with Am. geometri- 
cus. 
At the top of this basal shale, immediately below 
the massive limestone bed which contains Am. oblique- 
costatus., is a thin, but very interesting bed containing fish 
remains. 
Am. planicosta is common in the upper beds associated 
