376 ORTMANN — DISTRIBUTION OF DECAPODS [April S, 



lished by Scharff (1897). He distinguishes — aside from an Arctic 

 migration — two main routes of immigration into Europe during 

 the Later Tertiary period : 1, a southern one during Miocene and 

 Pliocene, which was directed from Western Asia over Asia Minor, 

 the Balkan Peninsula, Italy, Sicily, Algiers and Spain (and which 

 apparently sent a branch from the Balkan Peninsula into Central 

 Europe), and 2, a Siberian migration from Western Siberia through 

 Southern Russia to Central Europe, which belongs to the Pleisto- 

 cene (see map, /. c, p. 466) and was impossible before this time 

 (in Miocene and Pliocene), the Sarmatian and Ponto-Caspian Sea 

 forming barriers. 



Comparing our freshwater Decapods with the above, we see at 

 the first glance that the present distribution of the European fresh- 

 water crab, Potamon fluviatile, unmistakably agrees with that land 

 connection which began in the Miocene and culminated in the 

 Pliocene and which extends from Asia Minor over the Balkan 

 Peninsula to Italy, Sicily and Algiers. Even the minor features of 

 it are traceable. Potamon fluviatile is found everywhere in West- 

 ern Asia, in the Caucasus region and in the Crimea, but is missing 

 in the rest of Southern Russia. This corresponds to the fact that 

 the Crimea was connected in Pliocene times with the Caucasus and 

 was not in communication with the rest of Russia (see Scharff, 1897, 

 map p. 461). Potamon fluviatile is found in Asia Minor, Syria, on 

 the island of Cyprus and in Egypt. All these parts were then con- 

 nected. Along the tract of the land- bridge, from Asia Minor to 

 Italy and Algiers, this crab has been everywhere found. 1 This 

 relation of the supposed Pliocene land exteision with the distribu- 

 tion of Potamon fluviatile is so close that there is no objection 

 whatever to the assumption that the immigration of this species 

 falls in the Upper Pliocene, when this land connection was fully de- 

 veloped, and not in the Lower Pliocene, when there was only a 

 series of islands (see p. 374). 



Turning now to the crayfishes of Europe, we see that the centre 

 of the range of the group of Polamobius astacus (the Russian cray- 

 fishes) is just in that region which, during Miocene, Pliocene and 



1 That this species extended, in former times, farther to the north from the 

 Balkan Peninsula is shown by the discovery of it in iossil state in diluvial cal- 

 careous tufa near Süitö, Com. Komarom, Hungary (see Loerenthey, E., Nat. 

 Hefte Ungar. Nat. AIus., 1898. Review in Neues Jahrb. f. Mineral., etc., 

 1900, Vol. 2, p. 473). 



