460 Geological Society : — Dr. Agassiz on the 



Ganoidians, which comprehend above fifty extinct genera, and to 

 which we must refer the Plectognaths, Syngnathus, and Acipenser ; 

 thirdly, the Ctenoidians^ which are the Acanthopterygians of Cuvier 

 and Artedi, with the exception, however, of those which have smooth 

 scales, and with the addition of the Pleuronectes. Lastly, the Cy- 

 cloidians, which are principally Malacopterygians, but which com- 

 prehend besides all those families excluded from the Acanthoptery- 

 gians of Cuvier, and from which we must take the Pleuronectes 

 already placed in the preceding order. 



If we estimate the number offish now known to amount to about 

 eight thousand, we may state that more than three fourths of this 

 number belong to two only of the above-mentioned orders ; namely, 

 the Cycloidians and Ctenoidians, whose presence has not yet been 

 discovered in the formations inferior to the chalk. The other fourth 

 part of living species is referrible to the orders Placoidians and Ga- 

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

 during the whole period which elapsed since the earth 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 the class of fish more consider- 

 able differences 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, referable 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 fish 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- 



