THE LOWEE LIAS OE KEYNSHAM. 
47 
Cryptmnia consohrina^ Tate. 
Amherleya alpina^ Stol. 
Eucyclus aciiminatuSj Oh. and Dew. 
Turritella semiornata, Terq. 
Phasianella {Bourgetia) morencyana^ Piette. 
Troclms redcarensis^ Tate. 
Lamellibranchs . 
Ostrea : 
0. liassica, Strick. Form oval, pointed at the beaks. 
Beaks not curled either in or out of the general surface 
of the valve (i.e. without either Exogyra or Gryphea 
affinities), large valve flattened or convex, but not 
geniculate, small valve flat, outer surface composed of over- 
lapping lamellose growth margins, but marked by no radial 
folds. In the upper portion of its range it occurs with 
mutations of 0. irregularis. 
0. irregularis^ Miinst and Auctt. 
We have tentatively grouped under this name a number 
of forms, usually regarded as specifically distinct, but 
which we regard as mutations of one elemental type. One 
of us has already expressed the opinion that the elemental 
form, from which all the mutations are derived, is a true 
Gryphea., but distinct from G. arcuata.^ The unattached 
elemental type is very rarely found, and the mutations arise 
from variation in the area of attachment, combined with a 
more or less pronounced spiral curling of the beak to one 
side (after the manner of an Exogyra). 
This conclusion is in partial agreement with the views of 
Dr. Brauns, who considers the majority of the forms to be 
attached varieties of G. arcuata and G. cymhium, but 
separates those which show an Exogyra-Ydn^ character under 
the specific name of 0. ungula, Miinst. We cannot, however , 
convince ourselves that these views express the true re- 
Q.J.O.S., vol. Iviii. p. 721. 
