THE PROGRESS OF LIFE 63 



organisms. It is, therefore, quite possible to suppose 

 that protoplasm capable of secreting chlorophyll was a 

 later development, when the supply of mineral hydro- 

 carbons was getting exhausted ; and, consequently, the 

 first organisms may have been animals. 



ORDOVICIAN AND SILURIAN LIFE 



I now pass on to glance at the life of the Ordo- 

 vician and Silurian periods. The Ordovician was 

 ushered in by the appearance of the highest sub- 

 kingdom of animals, the vertebrata, represented by 

 minute teeth, called conodonts, from the green sands 

 at the base of the Ordovician, near St Petersburg. 

 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 

 Chsetopod worms ; others are thought to be of crus- 

 tacean origin, although no explanation is given of why 

 these crustacean jaws should always be found dis- 

 sociated 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 vertebrate 

 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 



