CEPHAIiOPODA. 313 



recent species ; in the deposits of a third or newer era (pliocene), 

 from 35 to 40 per cent., and, in still more modern formations 

 (pleistocene), the primitive forms have almost disappeared, 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 

 appear four or five of symmetrical figure, divided into chambers, 

 which are perfprated by a tube or siphon, like this large Nautilus 

 and this little Spirula, but 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 perishable soft parts. 



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

 Ammonites, Orthoceratites, Karaites, 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 in 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 branchiate Gasteropods, that have superseded them in the seas of the 

 tertiary and existing epochs. 



What, then, were the nature and affinities of the extinct con 

 structors of those ancient chambered siphoniferous shells ? Earnestly 

 and repeatedly had this question been pressed upon the attention of 

 zoologists and comparative anatomists, and long was it before anj' 

 satisfactory reply could be returned. The Nautilus Pompilius and 

 Spirula australis, which represent in the existing seas that vast 

 assemblage of siphoniferous Mollusca which peopled the ocean 

 during the secondary epochs, could alone yield the requisite data for 

 its determination, and for a long period comparative anatomists were 

 disappointed in their demands for these most rare and coveted sub- 

 jects. In fact, until the year 1832, geologists could be supplied only 

 with conclusions based upon more or less probable analogies and 

 conjectures. Before this period, only one account of the Nautilus 

 Pompilius was extant, in the work of Rumphius, a Dutch naturalist 

 of the 17th century, whose figure of the animal was pronounced by 

 Cuvier, the profoundest malacologist of his age, to be * indechiffrable.' 

 The little light that it might have thrown upon the interesting 



