4S 



,Ku|, ((,!( Al, 11>TI!I CI 'I 1 1 u\ AND I K \ K LI i|- M KNT. 



have been preserved in immense iiuinlH'rs. whilst llir more 

 fr:i;iile shells, such as Ar^onanta. do not aseend beyond tin- 

 pliocene; whether the naked eephalopoda or those provided 

 with a horny pen are ol' aneient origin \vc have no means of 

 determining.* 



Tin- approximate development of the genera in time may he 

 thus shown : 



The M[)j)earance and i-ange of the families of cephalopoda in 

 time may be similarly represented : 



Dilhvvn lias oitscrveii that the shells of camivorotiB gasteropods were 



almost or entirely wanting in tin- pahco/oir and secondary >t rata ; but 

 1 hey were rrplaeed. in t IK- economy of nature, by 1 lie almost extinct order 

 <t' tetra branchiate eeplialojiods -of A\ liich o\cr t \vo thousand species have 

 been described. 



