82 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.54. 



siderable number of collections. My records are from only 7 lakes 

 in the Boulder region and 11 near La Junta. To get a fairly ade- 

 quate notion of the fauna of the plains I have, accordmgly, had to 

 supplement my own records with all others available, chiefly those 

 of Beardsley from the Greeley region, and still our knowledge of 

 the plains fauna is less complete than of that of the mountains. 

 My own collections from lakes on the plains include 36 species. The 

 total list is 50, and 5 others, though collected only in the mountains, 

 are to be expected also on the plains, making a total considerably 

 larger than that of both mountain zones combined. Of these 55 

 species, 28 have been collected only in the plains, though a few of 

 these, on the basis of general distribution, are to be expected in the 

 mountains also. The remaining 27 range upward into mountain 

 zones. The few lakes collected in the Boulder region, just at the 

 border between mountains and plains, but really in the plains, seem 

 to have a fauna somewhat resemblmg that of the montane zone, 

 indicating, as is to be expected, that there is not a sharp dividing 

 line, and that these lakes belong as much to the mountains as to the 

 plains. 



Concerning the composition of the fauna it is unnecessary to go 

 into detailed description, as it is made up chiefly of species which 

 are common members of lowland faunas in America and to a con- 

 siderable extent in Europe and other Old World areas. This is 

 particularly true of the euthermic members of this fauna, but as 

 pointed out previously (Table 3), it is not true to a large extent of 

 the stenothermic members, those 28 species found only in the plains 

 area, of which 17 are confined to the western part of the United 

 States and 6 others to North America. This condition mdicates 

 that like the fauna of the mountain lakes, that of the plains is also 

 considerably specialized. This is a condition contrary to expecta- 

 tion, for we are accustomed to think of the plains conditions as the 

 "ordinary" and the mountains as the "exceptional" and so cal- 

 culated to produce the exceptional fauna. It appears, however, 

 that in the great plams of this country, especially their western 

 portion (probably on account of their arid climate) there exist con- 

 ditions of a quite specialized nature, differing decidedly from those 

 of lowland countries in general. This may furnish an explanation 

 for the restricted range of a considerable proportion of the species 

 of the plains zone in Colorado. A conspicuous feature of this fauna 

 is the large proportion of Phyllopods (12 species) confined exclu- 

 sively to the plains zone, none of which has a range extending beyond 

 the semiarid plams of western United States, northern Mexico, and 

 southern Canada, and most of them are much more restricted than 

 that. Though Phyllopods are universally distributed and every 

 portion of the world is hkely to have the group represented in its 



