180 Mr. Dilhn/n on Fossil Shells. [March, 



phaga, which feed only on dead animals. Without attempting 

 to distinguish the more predaceous from these other genera, I 

 shall however at present content myself with proving, and for 

 this I have adduced sufficient evidence, that the whole family of 

 the carnivorous Trachelipodes are extremely rare in all those 

 strata where the Ammonites and other Nautilida? abound. 



In describing the Ammonites, De Montfort, in his Conchologie 

 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 fence avec des 

 Comes d' Amnion." 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 Nautilidte still inhabit the southern 

 ocean, and their niolluscae 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 formation above the chalk, it therefore 

 appears, that the vast and sudden decrease of one predaceous 

 tribe has been provided 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 further 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 car- 

 nivorous 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 fur- 

 nished with an operculum are formed for the occasional respira- 

 tion of air; but i believe that this observation is not applicable 

 to the marine genera; and it was Adanson's opinion, that the 

 operculum is intended for the 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 ail the 

 marine Trachelipodes 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 protection of the testaceous Gasteropodes no such shield 

 would be wanting, and including this order it may be generally 

 observed, that none of the marine unoperculated Molluscs, 

 except the Cephalopodes, are to be found in the lias, or in any of 



