12 DISTRIBUTION OF FOSSILS. 



The geological distribution of animals and plants 

 varies considerably, some have a long range, others a short 

 one. Thus a few species extend through several formations, 

 as for instance, Alrypa reticularis from the Llandovery 

 Beds to the Devonian, and Orthis calligrarnma from the 

 Arenig to the Wenlock Limestone. Others have a short 

 range, as in the case of many graptolites and ammonites, 

 which are often confined to a single band of rock. Many 

 instances of genera with long and with short ranges will be 

 seen in the following pages ; of the former we may mention 

 Lingula extending from the Cambrian, and Nautilus from 

 the Ordovician, to the present day. Of the latter Belern- 

 nitella found only in the Upper Chalk, Coenograptus in 

 the Upper Llandeilo, and Tetragraptus in the Lower 

 Arenig. Often the genera which have a long range in 

 time have also a wide distribution in existing seas, as in 

 the case of Lingula, which lives off the coasts of Japan, 

 China, Formosa, the Korean Archipelago, the Philippine 

 Islands, the Sandwich Islands, Australia, and in the Indian 

 Ocean. 



With regard to the general succession of organisms, 

 we find, as might be expected, that those in the earlier 

 formations differ more from living animals and plants, 

 than do those in the later. Thus, in the Paleozoic, some 

 of the groups, many of the genera, and practically all the 

 species are extinct. But as we pass to more recent times 

 the proportion of extinct forms decreases, until in the 

 newer Cainozoic rocks almost all the genera and very 

 many of the species are still existing. The different 

 groups of organisms also appear in the geological series 

 in the order of their complexity of organisation, the 

 lower forms preceding the higher and more specialized. 

 Thus, taking the Vertebrata only, the Fishes first occur 



