xlviii. 



of the globe ; of the remainder, many range from the North 

 Atlantic to Australia, six are restricted to the North Seas, one to 

 the South, six to the South Atlantic, fifteen to the Pacific. 

 Thirty-six families live exclusively in fresh-water ; these present 

 many interesting peculiarities in their distribution. The Palae- 

 arctic and the Nearctic regions together contain twenty families. 

 The Palaearctic is characterised by the absence of osseous 

 Ganoidei, Cobitidae (Loaches), and Barbels. The Nearctic is 

 characterised by osseous Ganoidei, Amiurina, Cat-fish, and 

 Catostomina (Suckers) ; no Cobitidae or Barbels. Among 

 the peculiarities of distribution, is a curious fish found only in 

 Lake Baikal, in the mountainous district of Russian Siberia, 

 2,oooft. above sea-level and 1,000 miles from the sea ; its nearest 

 ally is the Mackerel (marine). Osteoglossum is represented by one 

 species, the huge Arapaima, in Brazil and Guiana, one in 

 Borneo and Sumatra, a third in Queensland. The curious 

 Lepidosirenida are represented by existing genera, Lepidosiren of 

 the Amazons, and Protopterus of the rivers of tropical Africa. 

 Fresh-water fish of the present fauna came into existence 

 during the Tertiary Age, when the great changes in the 

 distribution of land and sea took place. Salt-water is not in- 

 variably a barrier, and we may account for instances of singular 

 disconnections of families and genera. The dispersal of a single 

 type over several distant continents may and does generally point 

 to its great antiquity, but it does not prove that it is more so 

 than others limited to one region only. 



The Carboniferous and Permian Beds contain numerous fish 

 remains, allied to the Lepidosteus, the Gar-pike of North 

 America. Traces of Elasmobranchii, Sharks, and of Rays 

 in the Upper Silurians, become abundant in the succeeding 

 Devonian and Carboniferous Beds, and continue so to the 

 present day. The Ganoid, Ceratodus, which now lives in the 

 rivers and lakes of Queensland, reaches as far back as the Trias. 

 It is one of the very few instances where a genus, founded 

 upon the evidence of fossil specimens, has subsequently been 



If liniM in 'A \\\"\Y\c* t^f\r\ r\ if \ y-v-k TJ* *1 A il r- .1 



found in a living condition. Fossil teeth of the genus 



were 



