THE ORDOVICIAN PERIOD 



523 



and Australia, so that the range of the graptolite species was ocean- 

 wide. The history of individual species was not long, geologically 

 speaking, and hence the succession of species is well suited for 

 marking the progress of events in all parts of the ocean. During 

 the lifetime of the graptolites (limited to the late Cambrian, Ordo- 

 vician and Silurian), a score of successive zones, each characterized 



Fig. 385. The two upper curves represent the history of the trilobites accord- 

 ing to genera, the full line indicating the total number of genera, and the 

 dotted line the number of new genera introduced. The two lower curves 

 present the same data for the families of the trilobites, the full line repre- 

 senting the total number of families present, and the dotted line the 

 number of new families introduced. The data for the families is taken 

 from Beecher in the Zittel- Eastman text-book of Paleontology, Vol. I. 

 The data for the genera is somewhat incomplete, but is as full as can be 

 made from Zittel's " Handbuch der Palaeontologie." 



by particular species, have been identified. One of these zones 

 falls in the Cambrian, eight in the Ordovician, and eleven in the 

 Silurian. If these be taken as chronological bench-marks, the 

 successive horizons of the different continents may be correlated 

 accurately, and the progress of life in the various quarters of the 

 globe referred to a common standard. 



The Record of Marine Life 



The known fauna of the Ordovician was made up almost wholly 

 of marine invertebrates, among which trilobites and brachiopods 



