226 FISHES. 



Four small Indian Siluroids {Sisor, EretMstcs,Pscudecheneis, and 

 Exostoma) have been referred to the South American Hyposto- 

 matina ; but it remains to be seen whether this combination 

 is based upon a sufficient agreement of their internal structure, 

 or whether it is not rather artificial. On the other hand, the 

 occurrence and wide distribution in tropical America of a fish 

 of the Indian family Symbranchidte {Syvibranchus marmo- 

 ratus), which is not only congeneric with, but also most 

 closely allied to, the Indian Symbranchus hengalcnsis, offers 

 one of those extraordinary anomalies in the distribution of 

 animals of which no satisfactory explanation can be given at 

 present. 



4. The relation of the Indian region to the Tropical Pacific 

 region consists only in its having contributed a few species 

 to the poor fauna of the latter. Tliis immigration must have 

 taken place within a recent period, because some species 

 now inhabit fresh waters of tropical Australia and the South 

 Sea Islands without having in any way changed their specific 

 characters, as Lates calcarifer, species of Dules, Plotosus 

 anguillaHs ; others (species of Arius) are but little different 

 from Indian congeners. All these fishes must have migrated 

 by the sea ; a supposition which is supported by what we know 

 of their habits. We need not add that India has not received 

 a single addition to its freshwater fish-fauna from the Pacific 

 region. 



Before concluding these remarks on the Indian region, 

 we must mention that peculiar genera of Cyprinoids and 

 Siluroids inhabit the streams and lakes of its alpine ranges 

 in the north. Some of them, like the Siluroid genera Glypto- 

 sternum, Euglyptosternum, Pseudecheneis, have a folded disk 

 on the thorax between their horizontally spread pectoral fins ; 

 by means of this they adhere to stones at the bottom of 

 the mountain torrents, and without it they would be swept 

 away into the lower courses of the rivers. The Cypri- 



