190-'.] AND ANCIENT GEOGRAPHY. 291 



lowing opinion as to the origin of its distribution, founded exclu- 

 sively upon systematic and chorological studies. 



The oldest home of the Potamobiidce and their centre of origin is 

 somewhere in Eastern Asia. This ancestral stock spread chiefly in 

 two directions : a western extension of the range crossed Central 

 Asia, finally reaching Europe, while an eastern extension went 

 across Bering Strait and reached the western parts of North Amer- 

 ica. The continuity of this wide area, which was once wholly 

 occupied by the genus Potamobius, was interrupted subsequently in 

 Central Asia and where there is now Bering Sea, and thus three 

 isolated areas were formed — in Europe, in Eastern Asia and North- 

 west America. In each one of these parts the genus Potamobius 

 continued to develop separately. From the West American stock 

 of Potamobius finally issued the genus Cambarus, which probably 

 originated in Mexico and thence invaded the central and eastern 

 parts of North America. The origin of Cambarus probably lies far 

 back in time, since it shows no marked special affinities to any of 

 the three groups of Potamobius, and probably it was separated from 

 the latter genus before it was divided up into those three groups. 



2. Family Parastacidce Huxl. 



A systematic revision of this family has not been published hith- 

 erto. The present writer has tried to collect the necessary data for 

 a review in the " Thierreich," and although it is not possible to 

 give a complete synopsis, based upon careful criticism of the existing 

 descriptions as well as upon actual specimens, he has obtained a 

 fair general idea of the various forms which make up this family. 



According to these studies the present state of our knowledge of 

 the distribution of this group is the following : 



i. Genus Cheraps Er. em. Huxl. 

 Species : 



i. quinquecarinatus (Gr.). West Australia : Swan river. 



2. quadricarinatus Mrts. North Australia : Cape York. 



3. bicarinatus (Gr.). North and East Australia: Port Essington, 

 Cape York, Rockhampton, Burnett river, Sydney, Melbourne, 

 Murray river. 



4. preissi Er. Southeast Australia : Victoria. 



