56 



Dr. E. von Martens on the Occurrence of 



therein (taking for the Crustacea only Dana^s higher divisions in 

 -inea or -oidea, and not the families in -idee, in order to avoid too 

 minute a division), the number of families appears to be : — 

 Exclusively in Common 



Sea. Fresh water. to both. Total. 

 Fishes ... 16 16 23 55 



Crustacea . . 29 3 10 44 



Mollusca* . . 40 6 6 52 



Thus the number of the freshwater and marine forms is equal 

 only for the Fishes ; and even here, if we divide the Plagiostomi, 

 which were referred to for the sake of the general view only as 

 Rajce and Squali, into J. Miiller's 16 or 21 families, of which only 

 4 occur also in fresh water, the marine species acquire a prepon- 

 derance. 



According to the zones they are distributed as follows : — 



We see, consequently, that from the cold to the torrid zone 

 there is an increase of the common families; the exception of 

 the Crustacea in the torrid zone may only be apparent, and due 

 to our imperfect knowledge of the tropical freshwater animals. 

 This increase is not only an absolute one, such as is shown also 

 by the exclusively marine and freshwater families, and such as 

 was to have been expected, but a relative one, at the expense of 



[* The terra Mollusca here and in the following Tahles is used for 

 brevity to signify the Gasteropoda and Conchifera alone, these being the 

 only classes of Mollusca represented in both salt and fresh water.] 



