310 DISTRIBUTION AND ETIOLOGY UF THE CRAYFISHES. 



representation of the broad morphological differences 

 which mark off the Potamobiidce from the Parahtacidce. 

 Each group occupies a definite area of the earth's surface, 

 and the two are separated b}^ an extensive border-land 

 untenanted by crayfishes. 



A similar correspondence is exhibited, though less 

 distinctly, when we consider the distribution of the 

 genera and species of each group. Thus, among the 

 Potamobiidce, Astacus torrentium and nohilis belong essen- 

 tially to the northern, western, and southern watersheds 

 of the central European highlands, the streams of which 

 flow respectively into the Baltic and the North Seas, the 

 Atlantic and the Mediterranean (fig. 77, I.) ; A. leptodac- 

 tyliis, pachijims, angulosm, and colchicus, appertain to the 

 Pontocaspian watershed, the rivers of which drain into 

 the Black Sea and the Caspian (I.) ; while Astacus 

 dauricus and A. Schrenckii are restricted to the widely 

 separated basin of the Amur, which sheds its waters 

 into the Pacific (II.). The Astaci of the rivers of 

 western North America, which flow into the Pacific (IV.), 

 and the Ganihari of the Eastern or Atlantic water-shed (V.) 

 are separated by the great physical barrier of the Rocky 

 Mountain ranges. Finall}', with regard to the Par- 

 astacidce, the widely separated geographical regions of 

 New Zealand (VIII.), Australia (IX.), Madagascar (XII.), 

 and South America (VI. and VII.), are inhabited by 

 generically distinct groups. 



But when we look more closely into the matter, it will 



