NEOTROPICAL EEGION. 235 



INDIAN. NEOTROPICAL. 



Siluridae . 200 sp. Siluridse . 276 sp. 



Cyprinidae . 330 sp. Characinidae . 226 sp. 



Labyrinthici . 25 sp. Chromides . 80 sp. 



Ophiocephalidse 30 sp. Cyprinodontidse 60 sp. 



Mastacembelidse 10 sp. Gymnotidae . 20 sp. 



In both regions the great number of species is due to the 

 development of numerous local forms of two families, the 

 Characinidce taking in the New World the place of the 

 Cyprinidce of the Old World. Thereto are added a few smaller 

 families with a moderately large number of species, which, 

 however, is only a fraction of that of the leading families, the 

 remainder of the families being represented by a few species 

 only. The number of genera within each of the two regions 

 of the two principal famUies is also singularly alike ; the 

 Indian region having produced about 45 Siluroid and as 

 many Cyprinoid genera, whilst the Neotropical region is 

 tenanted by 54 Siluroid and 40 Characiaoid genera. These 

 points of similarity between the two regions cannot be acci- 

 dental ; they radicate that agreement in their physical and 

 hydrographical features which in reality exists. 



Of Ganoids, we find in Tropical America one species only, 

 Z&pidosiren 'paradoxa, accompanied by two Osteoglossoids 

 {Osteoglossum bicirrhosum and Arapaima gigas). 



Autochthont and limited to this region are the Polycen- 

 tridce, all the non- African genera of Ohromides and Ohara- 

 cinidce; of SiLuroids, the SypopUhalmina, Aspredinina, and 

 Stegophilina, and the majority of Pimelodina, Hypostomatina, 

 and Doradina ; the herbivorous Cyprinodonts or Limnophagce, 

 and numerous insectivorous Cyprinodonts or Garnivorce ; and 

 the GymnotMoe (Electric eel). 



The relations to the other regions are as follows : — 



1. The resemblances to the Indian and Tropical Pacific 

 regions partly date from remote geological epochs, or are partly 



