23 THE LESSON OF EVOLUTION 



Fossils called conodonts have been found in various 

 places, and in rocks of different ages, from the Upper 

 Cambrian to the Carboniferous, but they differ much 

 from one another. Some are, no doubt, the jaws of 

 Chaetopod worms ; others are thought to be of crus- 

 tacean origin, although no explanation is given of 

 W 7 hy these crustacean jaws should always be found 

 dissociated from the other parts of the exo-skeleton. 

 Possibly some may belong to Cephalopoda ; but the 

 conodonts, just mentioned , from St. Petersburg, have 

 been shown by Dr. J. von Rohon to have enamel and 

 dentine, with a pulp-cavity of an essentially verte- 

 brate character, and this has been confirmed by Dr. 

 Otto Jaekel ; so that in all probability they belonged 

 to an extinct order of lamprey-like animals. 



In the upper Silurian we find armour-plated Ostra- 

 codermi and Elasmobranchs , the latter represented 

 by fin-spines and small thorny scales. 



The invertebrates which first claim our attention 

 are the Graptolites and the Brachiopods. The Grap- 

 tolites are known in North America from the Upper 

 Cambrian to the Carboniferous ; but in Europe they 

 first appear in the Upper Cambrian as a monoprio- 

 nidian form allied to Dichograptus. In the lowest 

 beds of the Ordovician they suddenly attain their 

 greatest development, after which they gradually 

 declined, only a few forms pass into the 

 Devonian. The earlier forms had many irregular 

 which during the Ordovician period 

 in number, and became regularly 

 arranged ; while throughout the greater part of the 

 Silurian we find on]y simple, unbranched forms. 



