246 FISHES. 



tse-kiang in the east, and rivers flowing into the Adriatic, 

 Black and Caspian Seas, and Lake Aral, towards the centre 

 of this region. ISTone are known to have gone heyond the 

 boundaries of the Northern Zone. If the Lampreys are 

 justly reckoned among Preshwater fishes, their distribution 

 is unique and exceptional. In the Palsearctic region some of 

 the species descend periodically to the sea, whilst others 

 remain stationary in the rivers ; the same has been observed 

 in the Lampreys of ITorth America. They are entirely absent 

 in the Equatorial Zone, but reappear in the Temperate Zone 

 of the Southern Hemisphere. Many points of the organ- 

 isation of the Cyclostomes indicate that they are a type of 

 great antiquity. 



The remaining Palaearctic fishes are clearly immigrants 

 from neighbouring regions : thus Silurus, Macrones, and 

 Pseudohagrus from the Indian region ; Amiurus (and, as 

 mentioned above, Gatostomus) from North America. The 

 Cyprinodonts are restricted to the southern and warmer 

 parts ; all belong to the carnivorous division. The facUity 

 with which these fishes accommodate themselves to a 

 sojourn in fresh, brackish, or salt water, and even in thermal 

 springs, renders their general distribution easily compre- 

 hensible, but it is impossible to decide to which region 

 they originally belonged ; their remains in tertiary deposits 

 round the Mediterranean are not rare. 



B. The boundaries of the Nokth Ameeican or Neaectic 

 Eegion have been sufficiently indicated. The main features 

 and the distribution of this fauna are identical with those of 

 the preceding region. The proportion of Cyprinoid species 

 to the total number of North- American fishes (135 ; 339) 

 appears to be considerably less than in the Palaearctic region, 

 but we cannot admit that these figures approach the truth, 

 as the Cyprinoids of North America have been much less 



