A Period in the History of our Planet, 1 1 



logical development, notwithstanding the endless series of ca- 

 talogues of the petrifactions of the chalk, we possess unfortu- 

 nately no sufficient data ; yet the character of the cephalo- 

 pods especially is considerably modified, and the other fami- 

 lies of shells lose more and more the heterogeneous appear- 

 ance of the older epochs. The crustacca have no longer, as 

 formerly, the variety of extinct races ; in short, there is every 

 where visible a farther step towards the limit of hitherto at- 

 tained perfection. By dint of my various comparative inqui- 

 ries into the subject of fossil fish, I have succeeded in estab- 

 ishing, that in the development of the type of the vertebrated 

 animal, the chalk forms an essential march-stone, and the 

 separation is here more sharply carried out than in any other 

 formation. The ancient fishes were covered either with firm 

 bony scales, overlaid with enamel, similar to the scales of the 

 now existing Sauroids, Polyp teri, and Lepidostei, or their skin 

 was similar to that of Sharks and Rays ; it is in the chalk that 

 there first appear the representatives of the two great divi- 

 sions which now form the preponderating majority of species, 

 the fishes with horny, indented or entire scales, the Ctenoidians 

 and Cycloidians. But only a few species are found belonging 

 to genera which now inhabit our seas or rivers ; most are so 

 anomalously formed, that I was obliged to frame new genera 

 and families for them. 



But it is not in the class of fishes alone that this progress of 

 vertebrated animal development is manifested. Another very 

 important circumstance has recently been discovered, which 

 assigns to the epoch of the chalk an important part in the de- 

 velopment of creation. 



By a minute examination of the fishes contained in the cele- 

 brated Glaris slate, which was formerly considered a very old 

 formation, I have been able to demonstrate that it belongs to 

 the chalk ; and that it is only through alterations caused by 

 violent plutonic influences, that it has acquired its present 

 peculiar appearance. And these slates contain remains of 

 birds ! The feathered denizens of the air have left their tender 

 legs in these slates as evidence of their existence. It is to my 

 friend Escher von der Linth, the indefatigable student of the 

 Alpine formations, that we are indebted for this invaluable dis- 



