44 
THE LOWER LIAS OF KEYNSHAM. 
angles of whose meshes are the points most deserving of special 
names. Three such names are in common nse in the present 
group, viz., Am. bucklandij Am. conyheari and Am. hisul- 
catus. 
Am. conyheari^ Sow. denotes plate-like forms with very 
nnmerons whorls, a slow rate of growth and little involution ; 
very numerous short ribs, gently and simply curved, which 
die out quite gradually at the edge of the rim without any 
trace of a tubercle. 
Am,, bisulcatus, Brug. denotes thicker forms, with a 
much more rapid rate of growth and considerable involution ; 
very numerous large ribs which form a slight knob near the 
edge of the rim, and then bend sharply forward. 
Am. bucklandi, Sow. is an extreme form which grows 
rapidly in thickness, as well as in breadth, so that the 
section is almost square ; the ribs are far apart, low and 
broad, they die out gradually without a trace of tubercle 
near the edge of the rim. 
Of the three forms. Am. bucklandi is the most aberrant, 
and it seems difficult to understand how it came to be re- 
garded by Dr. Brauns, in Dev XJntere Jura as a middle 
form between Am. conybeari and Am. bisulcatus. (His 
numerical comparisons of the several forms take note of only 
two variable functions, viz., the width of the whorl and the 
diameter of the umbilicus.) 
All these three forms occur at Keynsham, but, of the 
three. Am. bucklandi is the rarest, and Am. bisulcatus by 
far the commonest. 
There is at least one other form which is common in the 
district, and easily recognized in the young, but since we do 
not know the adult, it cannot yet be named. The young 
form has the following characters : 
Diameter three inches per six whorls, only slightly em- 
bracing : whorl section rectangular, slightly taller than wide, 
