398 
Mr. Dillwyn on fossil shells. 
protection of the animal ; nor can I imagine any thing against 
which such a shield would be more necessary than the long 
and pliable fingers of the Cephalopodes, when they abounded 
in the seas, as they must formerly have done. It is, therefore, 
at least a curious coincidence, that all the marine Tracheli- 
podes of the transition and secondary strata, of which I can 
find any record, belong to genera which are furnished with 
an operculum, and that none of the numerous unoperculated 
genera should have been found in any other than the tertiary 
formations where the Ammonites disappear. For the pro- 
tection of the testaceous Gasteropodes no such shield would 
be wanting, and including this order it may be generally ob- 
served, that none of the marine unoperculated Molluscse, 
except the Cephalopodes, are to be found in the lias, or in any 
of its older strata ; and it appears to me that a much greater 
approach towards the same variety of testaceous animals 
which now inhabits our seas is to be found in the adjoining 
bed of lower oolite. 
The foregoing observations are confined chiefly to British 
fossils ; for as a few of the testaceous Cephalopodes still live 
in the warmer climates, it is possible that the Ammonites, as 
well as some others of the extinct genera may have existed 
longer, and that their remains may be found in the tertiary 
formations of the more southern latitudes. Although fossil 
Nautilidse are common in the secondary strata of the United 
States, they are said not to have been found in South America ; 
and it may therefore be queried whether the Cephalopodes 
were not confined to the more northern latitudes when the 
chalk formation was completed, and whether a decrease in 
the earth’s temperature at that period may not have occa- 
