344 Recherches sur les Poissons Fossiles. 



comprises four species all from Sohlenhosen. The fifth 

 genus, Pycnodus, Agass., contains one species from the mid- 

 dle Jura, Yorkshire, and Normandy ; four from the chalk of 

 Caen, Belgium, Kent, Maestricht, &c. ; two from Mount 

 Bolca ; and one from the Green Sand of Tilgate forest. 



The 4th family Gymnodonts, Cuvier, is composed of ge- 

 nera of the present creation, only one species being found 

 in a fossil state. 



The 5th family, Scleroderms, Cuvier, affords no in- 

 stance of extinct genera, and like the last, but a single in- 

 stance of extinct species. 



The 6th family, Lophobranchs, affords no fossil ge- 

 nus, and but two extinct species. 



Thus the 1st order of Fishes affords six families, the three 

 first of which are composed entirely of extinct genera, of which 

 above 120 species have already been described and figured 

 by M. Agassiz. The remaining families afford no instance 

 of an extinct genus, and but few of extinct species, even 

 in the newest and most superficial covering of the earth. 



The extinct genera would thus seem to be almost as nu- 

 merous and diversified as the living ; and appear to indicate 

 amongst themselves several distinct creations,, in as much as 

 those forms which are found in the old Red Sand stone, do 

 not exist in the rocks of the coal formation ; while those 

 which lived during the period of the coal formation, gradual- 

 ly became extinct^ and gave place to others which flourished 

 for a time, and in their turn also finally became extinct 

 during the Oolitic formation. These were followed by other 

 forms whose remains extend throughout the chalk. Still 

 more recent creations, approximating more to the character 

 of existing forms, are entombed in tertiary formations, and it 

 is only in the newest tertiary or most superficial layers of the 

 present surface, that the remains of existing genera are 

 found. 



(To be continued.) 



