100 



yet been discovered in the formations inferior to the chalk. The other 

 fourth part of living gpecies is referrible to the orders Placoidlans and 

 Ganoidians, which are now far from numerous, but which existed 

 during the whole period which elapsed since the eai'th began to be 

 inhabited, to the time when the animals of the greensand lived. 

 These remarkable conclusions to which M. Agassiz had come, from 

 the study of more than six hundred fossils on the Continent, have 

 been corroborated by the inspection of more than two hundred and 

 fifty new species found in English collections. 



The author next observes that in fishes more considerable dif- 

 ferences may be remarked within narrow geological limits than 

 among inferior animals. We do not see in the class of Fishes 

 the same genera, nor even the same families, pervading the whole 

 series of formations as takes place among zoophytes and testacea. 

 On the contrary, from one formation to another, this class is repre- 

 sented by very different genera, referrible to families which soon 

 become extinct, as if the complicated structure of a superior organi- 

 zation could not be long perpetuated without important modifica- 

 tions; or rather, as if animal life tended to a more rapid diversifica- 

 tion in the superior orders of the animal kingdom, during equal 

 periods of time, than in its lower grades. With respect to this, it is 

 with fishes nearly as with mammifers and reptiles, whose species, for 

 the most part but little extended, belong at a short distance in the 

 vertical series to different genera, without passing insensibly from 

 one formation to another, as is generally admitted to be the case 

 with certain shells. One of the most interesting facts which Mr. 

 Agassiz has observed is, that he does not know a single species of 

 fossil fish which is found successively in two formations, whilst he 

 is acquainted with a great number which have a very considerable 

 horizontal extent. But the class of fish presents besides to Zoolo- 

 gical Geology, the immense advantage of traversing all formations. 

 Thus they afford us the only example of a great division of verte- 

 brated animals in which we may follow all the changes experienced 

 in their organization during the greatest lapse of time of which we 

 possess any relative measure. 



The fish of the tertiary formations approach nearest to recent fish, 

 yet hitherto the author has not found a single species which he con- 

 siders perfectly identical with those of our seas, except the little fish 

 which is found in Greenland in geodes of clay, and whose geologi- 

 cal age is unknown to him. 



The species of the crag of Norfolk, the superior subapennine for- 

 mation, and the molasse, are related for the most part to genera 

 now common in tropical seas; such are the Platax, the large Car- 

 charias, the Myliobates, with large palatal plates, and others. In 

 the inferior tertiary formations, the London clay, the calcaire grossier 

 of Paris, and at Monte Bolca, a third at least of the species belong 

 to genera which exist no longer. The chalk has more than two 

 thirds of its species referrible to genera which have now entirely dis- 

 appeared. In it we already see even some of those singular forms 

 which prevail in the Jurassic series. But as a whole the fishes of the 



