58 Dr. E. von Martens on the Occurrence of 



opposite directions, as here, amongst the Fishes the common 

 Salmones, and amongst the Mollusca the exclusively freshwater 

 Limncecs and Pisidia prevail, the former protected from the frost 

 by their migrations, and the latter by their hybernation. 



E. There are families which are common to both media in 

 one zone and peculiar to one of them in another j of the four 

 cases possible here, we have in the 



Fishes. Crustacea. INIollusca. 



a. Exclusively marine in a colder "j 



zone than that in which they i- 17 6 5 



are common J 



b. Exclusively freshwater in the! 



cold zone j 



c. Exclusively marine in the\ {- 2 



warmer zone j 



d. Exclusively in fresh water in "I ■, 



the warmer zone J 



Of these the deficiency of the Blennioidei, Pleuronectidei 

 and Apodoidei, and perhaps also that of the Atherince, Idoteidea 

 and Cymothoidea in the fresh waters of the tropical zones, as well 

 as that of the Scicenoidea in the subtropical, and of the Leimceoidea 

 and Cyclopoidea in the cold zone, may be due solely to the defi- 

 ciency of our information, by which in a 2, and in c and d all 

 the examples would be cancelled. For b, the Silure in the Old 

 World furnishes a striking instance ; but in America there is in 

 the same zone a marine Silure [Galeichthys marinus, Mitch.). 

 The Siluroidei, and perhaps also Petromyzon, present the only 

 examples of families living especially in fresh water with indi- 

 vidual representatives in the sea ; the other common families 

 generally exhibit the opposite relation. 



Of the families included under «, the following first make 

 their appearance in the particular zones in 



II. III. IV. V. 



Of the Fishes 4 2 3 8 



Of the Crustacea 3 2 I 



Of the Mollusca 1 2 3 



This phsenomenon is therefore most remarkable and regular 

 amongst the Fishes {Gadini, Clupeoidei, Blennioidei, Lophobran- 

 chii) ; amongst Crustacea, according to our present knowledge, 

 it makes its appearance distinctly even in the temperate zone 

 {Caridea, Idoteidea, Cymothoidea), but amongst the Mollusca 

 only in the tropical zone : that it exhibits the greatest number 

 of examples in Zones 11. and V., is probably because II. to IV. 

 are merely subdivisions of the one temperate zone, so that only 

 II. and V. mark the occurrence of a new principal zone. 



From this we may formulate the following propositions foi* the 

 above-mentioned four classes : — 



