108 ASTACID^. 



are nearly all, except C. Blandingii and the Georgian C. penicillatus, rep- 

 resented at the mouth of the Mississippi. 



In Virginia, in the James River and its affluents, we find C. Bartonii 

 and C. robustus, with the southern form C. acutus. The first-named 

 species descends no farther than to the middle of the Mississippi ; the 

 second is of a decidedly northeastern type, more properly belonging to 

 the fauna characteristic of the waters that empty into the St. Lawrence. 

 In the more northern rivers, especially in the Potomac, Susquehanna, 

 Delaware, and their tributaries, we find C. Bartonii and C. affinis, and 

 perhaps C. obseurus. 



In the Hudson River, also in the other streams as far north as Ver- 

 mont, and in Lake Champlain, occurs C. Bartonii. But the mouth of 

 the Hudson River in New Jersey and New York is the extreme limit 

 of a peculiar variety of the southern species C, acutus, which is well 

 represented in North Carolina. 



The fauna east of the Alleghany Mountains is also Very peculiar. 

 Perhaps the most peculiar part is the well-explored and striking fauna 

 of Georgia. But we find farther to the north — besides C. acutus, 

 widely extended in the South, and C. Bartonii, a species to be found 

 in the whole middle part of the United States, — the exclusively north- 

 eastern species C. affinis and C. obseurus. 



The northern fauna, comprised in the immense water-basin of the 

 St. Lawrence and its tributaries, furnishes in Lake Superior, 0. virilis, 

 C. propinquus, C. rusticus, C. Bartonii ; in the Niagara, C. affinis and C. 

 propinquus ; in Lake Ontario and its affluents, especially Genesee River 

 and Lake Oneida, C. propinquus, C. obseurus, C. Bartonii, C. robustus, C. 

 obesus. Some of these species, and in fact all those found in Lake Su- 

 perior, as C. virilis, C. propinquus, C. rusticus, C. Bartonii, C. obesus, are 

 also represented in the regions watered by the Upper Mississippi and 

 its branches ; 0. obseurus, C. affinis, and C. robustus are the only species 

 peculiar to the northern fauna. The remarkable habitat of C. acutus 

 and C. obesus in Lake Michigan has been before mentioned. 



C. virilis occurs in the more northern waters, which empty into Hud- 

 son's Bay, especially in Lake Winnipeg, Saskatschavan, and Red River. 

 I am told that these waters are connected in the summer time through 

 marshes with the affluents of the Upper Mississippi. 



We also find true, especially for the genus Gambarus, that the United 

 States are divided into three great faunal regions, — the region traversed 

 by the Mississippi ; the eastern region, lying between the Alleghany 

 Mountains and the Atlantic coast ; and the northern region, which is 

 watered by the Northern Lakes and the St. Lawrence. I have not 

 spoken of the Mexican and Cuban species, my acquaintance with them 

 being as yet very imperfect. 



The three great regions just mentioned, particularly the first and the 



