THE LOWEE LIAS OF KEYNSHAM. 
41 
B. acuUts, Mill. Short guard with uniformly conical point , 
no apical grooves. 
Mutations : from a short simple' cone to more 
elongate cylindrical forms which are slightly 
compressed (B, infundibulum^ Phil, in part). 
Ammonites enter at the base of the Psilonotus zone and 
become prolific in the Arietes zone. 
Am. (Psiloceras) psilonotus j Quenst. includes both Am. 
planorbis^ Sow. and Am. johnstoni^ Sow. The two species 
agree in having smooth convex rims, wide umbilicus and 
flattened sides ; but, whereas Am. planorhis is smooth or 
transversely striate, Am. johnstoni has tall, short, straight 
ribs which only occupy the central portion of the whorl 
sides. In the Keynsham area neither species is found 
associated with the other nor with any other ammonite. 
Am. {ScJilothehnia) angulatus^ Schloth, 
The young form possesses sharp, simple, similar ribs 
which sweep sharply forward near the edge. The ribs from 
opposite sides either actually meet at an acute angle on 
the rim, or would do so if produced. There is considerable 
variation as regards the spacing of the ribs. 
There are two well marked mutations in the adult ; one 
in which the whorl sides are practically smooth, and the 
ribs are only represented by close set crenulations on the edge, 
and a second, in which the edges and rim are practically 
smooth, but the ribs persist as strong, well-spaced, straight 
ridges confined to the sides. A fairly complete specimen 
shows that the young of the first mutation has close-set ribs, 
most strongly pronounced at the edges, and it may be 
assumed that the second form is derived from a young stage 
which exhibits the characters of well-spaced ribs, most 
strongly developed on the whorl sides and less pronounced 
on the edges. 
Am. {Schlotheimia) charmassei^ d^Orb, is recognized by 
