397 
Mr. Dillwyn on fossil shells. 
Ill describing the Ammonites, De Montfort, in his Con- 
chologie Systematique, observes, that they are found of all 
sizes, “ depuis la grandeur d’une Lentille jusqu’a celle de 8 
pieds de diametre and, as a proof of their great abundance, 
Lamarck says, “La route d’Auxerre a Avalon, en Bourgogne, 
est ferree avec des Cornes d’Ammon.” These Ammonites, 
as well as most of the other principal multilocular genera, 
appear to have become extinct in our northern latitudes when 
the chalk formation was completed ; but a few of the Nau- 
tilidce still inhabit the southern ocean, and their molluscae 
belong to the carnivorous order which Lamarck has described 
under the name of Cephalopodes. From the occurrence in such 
great numbers of the carnivorous Trachelipodes in the for- 
mation above the chalk, it therefore appears, that the vast 
and sudden decrease of one predaceous tribe has been pro- 
vided for by the new creation of many genera, and a myriad 
of species possessed of similar appetencies, and yet formed 
for obtaining their prey by habits entirely different from those 
of the Cephalopodes. 
It may be farther observed, that all the marine genera of 
the herbivorous Trachelipodes to which either of the fossil 
species belongs, are furnished with an operculum, and that 
the few carnivorous species which have been found in the 
secondary strata, agree with them in this particular, although 
the unoperculated genera are very abundant in the London 
clay. Lamarck, of the fresh water Trachelipodes says, that 
those which are not furnished with an operculum are formed 
for the occasional respiration of air ; but I believe that this 
observation is not applicable to the marine genera ; and it was 
Ad anson’s opinion, that the operculum is intended for the 
