14.5 
AMMONITES Plicomphalus. 
TAB. CCCCIV. 
This figure represents the young state, or what 
amounts to the same thing, the inner whorls of an Am- 
monite formerly given at tab. 359, but has so different 
an appearance, that it w ould not be suspected to be the 
same, were they not found united in one specimen, which 
has been proved by breaking the one first figured ; and 
shew s the necessity of adding to the specific character 
that the front is, in the young state transversely fur- 
rowed , This specimen is also from Boling-broke. 
AMMONITES mutabilis. 
TAB. CCCCV. 
Spec. Char, Depressed; outer whorls com- 
pressed, plain and smooth ; inner whorls 
two thirds exposed, tuberculated, plicated ; 
plicae interrupted over the front ; aperture 
ovato- sagittate. 
In the young state, the front of this Ammonite, has a 
narrow, battened space around it, from which numerous 
very neat plicae spread half way over the sides ; next to 
these, a single row of compressed tubercles is placed, 
the inner margin of the whorl is smooth : when the shell 
is about two inches in diameter, the tubercles cease to be 
formed ; but the plicae continue until it is six inches 
over, when they also gradually disappear, the front be- 
comes rounder, and the sides quite plain, and com- 
pressed rather obliquely towards the front, so as to give 
the aperture a blunted sagittate form ; the septa are 
rather close, and sharply sinuated ; the shell pearly; 
there are at distant intervals contractions in the whorls, 
that have been probably formed by an inflected or thick- 
ened lip, at successive periods of growth. 
The figures represent a small individual shewing the 
ornamental part of the shell, and a portion of a large 
specimen, measuring ten inches in diameter, and of 
which the greatest thickness is an inch and an half; I 
am indebted to George Wier, Esq. for them, they were 
produced by the Clunch Clay near Horncastle. 
There appears to be no regular rule amongst Am- 
monites for their change of form, some becoming more 
