THE APTYCHUS. 113 
sitic shells; consequently other evidence is necessary to 
establish their connexion, and a few links in that chain of 
evidence I hope to add. 
In the upper lias of the neighbourhood of Ilminster 
the Aptychus is frequently found, and in a condition 
and state of preservation which probably allows of its 
being better studied than in many other beds. In- 
terposed between some beds of clay, containing a very 
interesting group of organic remains, there occurs & 
thin bed of yellow limestone, in the breaking up of which 
it often happens that a section of some small ammonites 
are obtained, and a fortunate fracture occasionally gives a 
view of the Aptychus lying in the outer chamber. It is 
also occasionally found in another way in the same bed. 
Now and then ammonites of a larger size were imbedded ; 
and when this was the case, they appear to have been 
acted upon by water, which passing gently over them, 
(probably before the bed had become much hardened,) 
tended to facilitate the decay of the shell, leaving in many 
instances nothing but their casts. Curiously enough, in 
these casts the Aptychus is frequently left in the most 
perfect preservation, lying in that part of the cast that 
would answer to the outer chamber of the ammonite, 
generally symmetrically placed, and always corresponding 
to the size and growth of the shell they appear to have 
inhabited, or in which they are found. These casts also 
shew that before the ammonites were covered up, there 
were attached to them the parasitic shells I mentioned ; 
oysters and other shells appear at times to have covered 
all parts of them. But there is this to be noticed, that 
these shells are not so often found in the interior of the 
chamber, as attached to the outside ofthe shell ; and that 
whilst there may be a hundred oysters, there is never more 
1851, PART II. p 
