THE TRILOBITA AND OTHER ARTHROPODA 213 



Besides their value as indices of age, trilobites may 

 serve another purpose to the geologist. The majority 

 being bottom-living forms (benthic) they were more re- 

 stricted than pelagic forms in geographical extension. 

 They may therefore assist in the delimitation of marine 

 zoological provinces in Palaeozoic times, and so help 

 towards the reconstruction of past geographies. Such 

 provinces are regarded by some geologists as established 

 for the Cambrian, Ordovician, and Devonian periods, 

 though the Silurian gives little indication of provinces. 

 Thus, in the Cambrian of China there are trilobites 

 belonging to genera unknown in Europe, and again in 

 the Rocky Mountains there is another series of forms, 

 while in Newfoundland the European faunas are found. 

 There is therefore a probability of separate provinces 

 between which migration was difficult/* At the same 

 time great caution is necessary in drawing conclusions 

 from limited data. Evidently if two faunas from different 

 parts of the world contain no species in common, we 

 cannot be sure that they were contemporaneous : it 

 may be that they belong to different zones in the system, 

 and that one zone was never deposited in the one area, 

 or was subsequently removed by denudation, while the 

 same was the case with the other zone in the other area. 

 Mistakes of this kind have been made and afterwards 

 corrected in several cases, and it may be that the same 

 will happen to the supposed Palaeozoic life-provinces. 



As more detailed knowledge is obtained of the strati- 



i 



* See Cowper Reed, " Pre-Carboniferous Life Provinces.' 

 Records Geol. Surv. India, vol. xl. (1910). 



