1902.] AND ANCIENT GEOGRAPHY. 273 



The following groups of freshwater Decapods are known : 

 Family: Atytda. 



Palcemonidce (in part). 



PotamobiidcB. 



ParasiacidcB. 



jEgleidce (monotypic). 



Potamonidce. 



There are, scattered among other families, other forms of fresh- 

 water Decapods, but the above are the most important groups. 

 These are found either exclusively in fresh water or possess the 

 largest number of their members there, and are found only in rare 

 cases in the sea. 



As regards the Atyidce, the present writer has collected the cho- 

 rological material in a previous paper.^ This is no doubt one of the 

 oldest groups of freshwater Decapods, and their origin, as is very 

 likely also according to their morphological characters, is to be 

 sought for possibly in Jurassic times, although fossil formxS are not 

 positively known. The chief features of their distribution are 

 excessively abnormal and even confusing, and therefore the extreme 

 age of the group is again confirmed. On the other hand, there are 

 smaller groups within this family, the distribution of which was 

 apparently formed in later times. Since there is every reason to 

 believe that our knowledge of the actual distribution of the AtyidcF. 

 is still more or less defective, we shall refrain from discussing it and 

 refer only to the latest summary given by the present writer.' 



In the family of the PalcemomdcB the genus PalcBmon forms a 

 group that possesses numerous species which are found chiefly in 

 fresh water. Their distribution, which has also been previously 

 investigated by the present writer,* points distinctly to the fact that 

 this genus is a very recent one, which is at the present time just in 

 the act of immigrating into fresh water, and that this process is by 

 no means completed. The different species depend in their dis- 



1 Compare Ortmann, A. E., in Bronn's Klassen und Ordnungen des Thier. 

 reichs. Vol. v, 2, 1899, p. 1^85. We leave out of consideration the families 

 Coenobitidce and Gecarcinida:, which are more properly land animals. See ibid., 

 pp. 1 183 and 1 184. 



2 Ortmann, in Proc. Acad. Philadelphia, 1894, p. 397 ff. 



3 In Bronn's Klassen tend Ordn., I. c, 190 1, p. 1286 f. 



^ In Zool. Jahrb. Syst.,NQ\. v, 1891, pp. 744-748, and in Bronn's Klassen tend 

 Ordn., I. c, 190I, p. 1291 f. 



