48 
THE LOWER LIAS OF KEYHSHAM. 
lationsliip of the very numerous forms which have come 
under our notice ; the transition from a form which pos- 
sesses considerable curling of the beak to one which shows 
none at all seems to us to be perfectly continuous. Again, 
though the Exogyra character is often strongly pronounced 
in the large attached valve, the upper flat valve usually 
exhibits no greater lateral displacement of the beak than 
is to be seen in 0. flahellula^ from the Eocene, and other 
kindred forms which are entirely removed from the true 
Exogyras. The curling of the beak in the large valve 
would seem to be, in part at least, directed towards securing 
that stability of attachment which is jeopardized by the 
large convex area, exposed to the action of forces of dis- 
placement. 
Since variation proceeds, on the one hand, towards Ostrea 
liassica, and, on the other, towards a typical Gryphea 
whilst the maximum development of 0. ^pularis^ in its 
most characteristic shape, takes place between the periods 
at which these two forms respectively flourished, it seems 
a natural suggestion that the present species supplies the 
necessary link between them. 
O. anomala^ Terq. and 0. marmorai^ J. Haime, seem to us 
to be mutations of the same general type. 
The Alectryonia division is represented by 0. multicostata, 
Terq. (non Miinst) and 0. semiplicata, Miinst ; the Gryphea 
division, by G, arcuata^ Lam., with which 0. irregularis is 
often associated. 
Anomia : 
A. pellucida^ Terq. is very common at the base of the 
Semicostatus zone. 
Small Anomias occur in the Echinid clay. 
Plicatula : 
P, intusstriata^ Emm, is not very common. We have 
