48 



DISTRIBUTION AND I > K V KU "I'M KN 'V . 



have been preserved in immense numbers, whilst the more 

 fragile shells, such as Argonauta. do not ascend beyond the 

 pliocene ; whether the naked cephalopoda or those provided 

 with a horny pen are of ancient origin we have no means of 

 determining.* 



The approximate development of the genera in time may be 

 thus shown : 



The appearance and nmge of the families of cephalopoda in 

 time may be similarly represented : 



x Dillwyn lias observed tliat the shells of carnivorous .^asteropods were 

 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 tetrabranehiate cephalopods of which over two thousand species have 

 been described. 



