226 ZOOLOGY SECT. 



strongly compressed, perfectly transparent, and have colourless 

 blood. They are sometimes known as " Glass-fish/' and were 

 formerly placed in the genus Leptocephalus, their real nature being 

 unknown. The Crossopterygii (or at least Polypterus) are unique 



Fio."S49. Polypterus bichir. Head of advanced larva ; E. G. external gill. (From Dean, 



after Steindachner.) 



in the sub-class in possessing, on each side, a single external gill, 

 as in Dipnoi and Amphibia (vide infra). 



The Geographical Distribution of the Ganoid Teleostomi is 

 curiously limited : they are all essentially fresh-water forms- 

 although some Sturgeons are found in the sea and are almost 

 exclusively inhabitants of the Northern Hemisphere, and especially 

 of the Holarctic Region. The Chondrostei occur in the rivers of 

 Europe, Asia, and North America: one genus of Sturgeons 

 (Scaphirhynchus) lives in the Mississippi and in the rivers of 

 Central Asia, but not in the intermediate regions: in the same 

 way Polyodon is found only in the Mississippi, while the closely- 

 allied Fsephurus is found in the Yangtse-kiang and Hoangho a 

 striking instance of discontinuous distribution. Amia is found 

 in the fresh waters of the United States; Lepidosteus extends 

 also into Central America and Cuba. Polypterus lives in the 

 Upper Nile and some other tropical African rivers ; Calamoichthys 

 in the Old Calabar River. 



Among Teleostei the Physostomi are largely, though not ex- 

 clusively, fresh-water Fish; the Carps, Eels, Salmonoids, and 

 Siluroids are important examples. The Acanthopteri, Pharyngo- 

 gnathi, and Anacanthini are mostly marine, some being in- 

 habitants of the shores, some pelagic, some abyssal, extending 

 to a depth of nearly 3,000 fathoms. As we have seen, many 

 species are practically terrestrial. All the sub- orders are uni- 

 versally distributed, so that we have to descend to families before 

 meeting with any important facts in geographical distribution. 



The Distribution in Time of the Teleostomi is interesting 

 as showing the gradual replacement of the lower or more 

 generalised members of a group by the higher or more specialised 

 forms. During the whole of the Palaeozoic and the greater part 

 of the Mesozoic era the three orders of Ganoids, to-day small 

 and isolated groups, formed the whole of the Teleostotnian fauna, 



