52 Dr. E. von Martens on the Occurrence of 



Crabs, existing in the sea as far as Greenland and Spitzbergctt, 

 is only represented in the fresh ^vater in the subtropical climates 

 by one Thelpliusa, and rises in the West Indies by the Gecarcinus 

 to a permanent dwelling on land. Amongst the bivalve Mol- 

 lusca, Area scaphulu, Benson, lives in the Jumna near Humer- 

 poor, at a distance of 1000 English miles from the sea, and 

 Pholas rivicula, Son%, in the fresh water of the river Pantai, twelve 

 English miles above its mouth, in floating wood. Thus our 

 usual notion of the distribution of marine and freshwater ani- 

 mals in different families, derived from the circumstances of our 

 native country, is more and more modified with the advance of 

 knowledge, and it becomes a question which of the numerous 

 famiUes of aquatic animals are exclusively proper to one of the 

 two media, and how far the dwelling-place is in accordance with 

 the systematic position, that is to say, with the modifications 

 of organization, — a question which is of peculiar geological 

 interest. To arrive at a result which shall not be entirely 

 negative, it will be advisable to pay no regard at all to the 

 multifarious mixtures and points of transition between the two 

 elements, such as are presented on a small scale by the mouths 

 of rivers and saline lakes, and on a large scale l)y the Baltic and 

 Caspian Seas, and to confine ourselves solely to the contrast of 

 rivers and inland lakes with the open sea. We must also dis- 

 regard the distinction of the families whose species all live in 

 the sea, and of which some only ascend temporarily into the 

 fresh water (such as Alosa), from those which possess a few con- 

 stant representatives in the latter medium (e. g. Lota), as with 

 regard to many, and some of these the most interesting of the 

 exotic river-fish, we do not kno^v whether they are migratory 

 or stationary. "With these limitations, and the still more im- 

 portant ones of our present knowledge, the following Table 

 furnishes a summary of the freshwater animals amongst the 

 Fishes, Crustacea and Mollusca, according to families and cli- 

 matic zones, especially for the Old World, in which, however, 

 those occurring only in the other hemisphere are inserted, with 

 the corresponding indication, N. Am., S. Am., or Austr. (North 

 Amei'ica, South America, Australia). With regard to its fresh- 

 water animals, Iceland is related to the countries of the high 

 northern latitudes; Central Europe is considered to extend 

 southwards to the principal range of the Alps ; Egypt and Syria 

 (on account of Mastacemblus) are referred to the torrid zone. 



0. Indicates that the family does not occur in this zone. 

 m. Indicates that the family only occurs in the sea in this zone. 

 — . Indicates that the family occurs both in the sea and in 

 fresh water in this zone. 



