|S t.KnUMiir.M, msTKIUI Tlo.N AND l> K V KM U'.M KXT. 



have leen preserved in immense numbers, whilst the more 

 fragile >hells. such MS Argonanta. do not ascend beyond the 

 pliocene ; whether the naked cephalopoda or those provided 

 with a horny pen are of ancient origin we have no mean> <>!' 

 determining.* 



The approximate development of the genera in time may lie 

 thus shown : 



The appearance and range of the families of cephalopoda in 

 time may be similarly represented: 



* l)ill\\ YM has ohscrvrd Unit the shells of rarnivorous pastel <'],(. .Is \\on- 

 almost or entirely wanting in the palaeozoic and secondary strata ; but 



they were replaced, in the economy of nature, by the almost extinct order 

 of tetrabrandiiatc cc)>lial<>)>o<ls -of \\hicli oxer two thousand species have 

 l.een described. 



