236 



ZOOLOGY 



SECT. 



exclusively inhabitants of the Northern Hemisphere, and especially 

 of the Holarctic Kegion. 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 Psephurus 

 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. 



B 



nch 



FIG. 911. .4, restoration of Glyptolepis (Devonian) ; B, Macropoma mantelli (Cre- 

 taceous), a. bl. ossified air-bladder ; d.f. 1, d.f. 2, dorsal fins ; /*. a. haemal arches ; jug. pi. 

 jugular plates : . a. neural arches ; nch. position of notochord ; pet. f. pectoral fin ; pv.f. 

 pelvic fin ; ./. ventral fin. (From Nicholson and Lydekker.) 



Among Teleostei the Physostorni 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 inhabitants 

 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 universally distri- 

 buted, so that we have to descend to families before meeting with 

 any important facts in geographical distribution. 



The Distribution in Time of the Teleostonii is interesting 

 as showing the gradual replacement of the lower or more generalised 



