NOETHEEN ZONE. 241 



main with the northern limit of the Equatorial Zone ; but at 

 three different points they overlap the latter, as has been 

 already indicated. This happens in, and east of, Syria, where 

 the mbced faunae of the Jordan and the rivers of Mesopotamia 

 demand the inclusion of this territory into the Northern Zone 

 as well as the Equatorial ; in the island of Formosa, where a 

 Salmonoid and several Japanese Cyprinoids flourish ; and in 

 Central America, where a Zepidosteus, a Cyprinoid (Sclerog- 

 nathus meridionalis), and an Amiurus {A. meridionalis) repre- 

 sent the North American fauna in the midst of a host of 

 tropical forms. 



A separate Arctic Zone does not exist for Freshwater 

 fishes; ichthyic life becomes extinct towards the pole as 

 soon as the fresh water remains frozen throughout the year, or 

 thaws for a few weeks only ; and the few fishes which extend 

 into high latitudes, in which lakes are open for two or three 

 months in the year, belong to types in no wise differing from 

 those of the more temperate south. The highest latitude at 

 which fishes have been obtained is 82° lat. N, whence the 

 late Arctic Expedition brought back specimens of Charr 

 (Salmo arcturus and Salmo naresii). 



The ichthyological features of this zone are well marked : 

 the Chondrosteous Ganoids or Sturgeons, and the families 

 of Salmonidm and Usocidce are limited to, and characteristic 

 of, it ; Cyprinoids flourish with the Salmonoids, both families 

 preponderating in numbers over the others, whUst the Silur- 

 oids are few in number and in variety. 



The two regions in which this zone is divided are very 

 closely related, to one another, and their affinity is not unlike 

 that which obtains between the sub-regions of the Southern 

 Zone. The subjoined list will show their close agreement 

 with regard to families as well as species. Several of the 

 latter are common to both, viz. — Acipenser sturio, A. macu- 

 latus, Perca fluviatilis, Gastrosteus pungitius, Salmo solar, 



E 



