THE LOWER LIAS OF KEYNSHAM. 
43 
strongly tuberculatej the other only slightly so or entirely 
non-tnberculate. There is little doubt that the two sections 
are connected by several transitional forms, but the vast 
majority of specimens fall readily and unquestionably into 
one section or the other. In each section the mutations 
proceed on the same lines. 
A. — The tuberculate section (group of Am. rotiformis). 
Of this section the two following forms are the commonest 
in the Keynsham area : — 
Am. rotiformis.^ Sow. is a broad, plate-like ammonite 
with very numerous whorls ; the ribs are close-set and 
nearly straight, and end in tubercles near the edge ; the 
width of the whorl is usually greater than its thickness. 
Am. coronaries^ Quenst. has well-spaced, strongly-curved, 
coarse ribs, which develop teat-like tubercles near the edge. 
The whorls, which only just embrace, are strikingly narrow 
compared to their thickness. 
B. — 'The non~tuberculate section (group of Am. hisulcatus). 
This group includes by far the greater number of in- 
dividuals. 
The ribs are gently curved, and either die out gradually 
on approaching the rim or develop a non-prom inent point, 
where they bend rapidly forward, at the edge. 
Between any two selected individuals there is every 
possible mutation in regard to the rate of growth, in the 
number and curvature of the ribs and in the amount of 
involution. 
Whilst recognizing to the full the complete passage of one 
form into another, it is nevertheless convenient to have 
names to indicate the most commonly occurring stages in 
the mutation. 
The mutation does not proceed along a single line, de- 
termined by the variation of a single character, but rather 
the whole group must be represented by a network of lines, the 
