THE EUROPEAN SEAS. 



235 



The northern coasts of Massachusetts have Tes- 

 tacea, of which one-half are common to our Euro- 

 pean side of the Atlantic, and which belong to our 

 " Boreal " province. These two opposite sections 

 are only isozoic in degree, but they are equivalent, 

 and may be called Omoiozoic. 



As northern forms decrease in number from north 

 to south along both sides of the Atlantic, the pro- 

 portion of common species decreases ; still a corre- 

 spondence is maintained by representative forms 

 rather than by identical ones, and the system of 

 omoiozoic zones is continued even when, as in the. 

 case of the Canaries and the Antilles, there should 

 be only two species in common. 



The application of such considerations as these by 

 the palaeontologist to his own special inquiries is 

 easy and interesting. There was no greater amount 

 of uniformity in past times than there is at present ; 

 distribution has ever been influenced by the same 

 laws.. If on investigating old sea-beds it shall be 

 seen that there are areas over which the fauna is 

 uniform or isozoic, whilst in other directions it 

 presents change, we shall be justified in seeking the 

 explanation in the causes which produce like results 

 at present. If, for instance, the localities of the 

 great Saurians (Enaliosaurs) — the monsters of the 

 secondary seas — are found to be northern and tem- 

 perate, but not southern, we may be allowed to infer 

 that the distribution of these forms was somewhat 

 that of our existing Cetaceans, and that they be- 



