4 V5 
Mr. Dillwyn on fossil shells. 
such perforation in either of those species which are found 
in the more regular beds of green sand, and which are some- 
times mixed with them. These perforations may be readily 
distinguished from those more oblique and lateral burrow- 
ings which are often found in secondary fossils, and are al- 
ways conveniently formed for suction by being broadest in 
the outer surface, and go straight through that part of the 
shell which is immediately over the animal ; whereas in the 
latter the holes are cylindrical, and much more resemble 
the indiscriminate burrowings which are common in recent 
oyster shells. 
In my former Letter, which the Royal Society has done me 
the honour to publish in the Philosophical Transactions of last 
year, I have pointed out some of the changes which took 
place immediately after the chalk formation was completed ; 
and of the British strata it may be further observed, that it is 
only in the tertiary beds that any traces of the cirrhipeda, or 
of any of the families of naked mollusca have been found. 
The beak, which has been figured by Blumenbach, and 
which among the fossils of the lias is mentioned by Cony- 
beare and Phillips as the beak of a sepia, belonged, as I 
think, unquestionably, to the cephalopode animal of an am- 
monite ; and it sufficiently resembles the lower mandible of 
the parrot-like beak which Rumphius has described of nau- 
tilus pompilius. As might be expected, if these mandibles, 
or rather casts of mandibles, belong to the ammonites, they 
differ generically in shape from those of every living genus 
of chephalopoda which has been figured or described, and I 
have found them in all those beds; and, so far as I can 
