576 LECTURE XXIII. 



LECTURE XXIII. 



CEPHALOPODA. 



We trace the progressive diminution of the existing species of Gas- 

 tropodous Mollusca by their fossil shells through the descending 

 strata of the tertiary periods of geology, beyond which such indi- 

 cations become very doubtful and obscure.* In the oldest tertiary 

 deposits, not more than 3J per cent, of the remains of any class of 

 Mollusca have been identified with species now living. From this 

 fact, which indicates the dawn of the existing state of the testaceous 

 fauna, the term " eocene " is applied to these strata : in the superim- 

 posed or " miocene " tertiary beds there are about 17 per cent, of fossil 

 shells, identical with recent species ; in the deposits of a third or 

 " pliocene " era from 35 to 40 per cent. ; and, in still more modern 

 " pleistocene " formations, the older tertiary shells have almost dis- 

 appeared, and the number of species identical with those now living 

 is from 90 to 95 per cent. 



Amongst the shells which characterise the eocene strata, there 

 are four or five of symmetrical figure, divided into chambers, which 

 are perforated by a tube or siphon, like the existing large Nautilus 

 and the little Spirula ; but they belong to species which are unknown 

 in modern seas. In the secondary formations, which succeed the 

 eocene in depth and order of antiquity, the chambered siphoniferous 

 shells become more numerous and diversified ; they depart further 

 from the two remaining recent types, and manifest a rich variety of 

 form and structure. From these modifications of the shells we not 

 only infer corresponding differences in the habits of their extinct 

 occupants, but we can trace, in some instances, the nature of the 

 associated differences in the organisation of the soft parts. 



The vast number of the complex shells known by the names of 

 Ammonites, Orthoceratites, Hamites, Baculites, Turrilites, Belem- 

 nites, &c., and the constancy which particular genera and species 

 manifest in their relations to particular strata, indicate that the 

 functions which their molluscous fabricators performed ix^ the organic 

 economy of the ancient world must have been equal and closely 

 analogous to those which have since been assigned to the Pectini- 

 branchiate Gastropods that have superseded them in the seas of the 

 tertiary and existing epochs. 



* The supposed recent species of Trochus observed by Defrance in chalk, and 

 the Paludina and Cyclas, described by Dr. Fitton from the Wealden, are not con- 

 sidered to be identical with existing species by Deshayes and Lyell. 



