NO. 2226. DISTRIBUTION OF ENTOMOSTRACA—DODDS. 75 



their distribution. Thus we recognize on the basis of latitude a 

 very striking zonation of both animal and plant life and in a similar 

 way a definite zonation on the basis of altitude. In the region of the 

 Rocky Mountains from which my collections were made, Ramaley 

 (1907) has defined and limited the plant zones on the basis of dis- 

 tribution of forest growths as follows: 



Plains zone: Up to 5,800 feet. Grassland with trees and shrubs 

 along water courses only. 



Foothill zone: 5,800-8,000 feet. An open forest chiefly of rock 

 pine iPinus scopulorum) . 



Montane zone: 8,000-10,000 feet. Close forest of lodge-pole 

 pine {Pinus niurrayana) . 



Subalpine zone: 10,000-1 1,500 feet. Forests chiefly of Engelmaim 

 spruce (Picea engelmanni) . 



Alpine zone: Above 11,500 feet Above timber fine, where con- 

 ditions are so extreme that trees will not grow. 



Such a zonation gives a definite datum to which other animals or 

 plants may be referred. This expresses much more significantly their 

 true position than to place them between such and such elevations, 

 because on the basis of zonal position we at once have suggested the 

 environmental conditions and associates. 



A study of the plankton Crustacea in my collections, and compari- 

 son with the records of others, so far as they can be applied to this 

 problem, indicate three pretty well defined zones, which I shall call 

 alpine, montane, and plains zones, the first and the last correspondmg 

 in the main with Ramaley's zones of the same names, and the montane 

 zone covering prett}^ much the same range as his three middle ones. 

 Whether these shall prove to be of general application to other parts 

 of the Rocky Mountains or not, they afford an mstructive method of 

 pointing out the significant features of distribution in the region under 

 study. 



Alpine zone. — This zone includes lakes, nearly all of which are 

 above 11,000 feet. The bodies of water that I have assigned to this 

 zone (43 m all) include the lakes that I have designated as "alpine" 

 lakes (together with a very few of these on upper stream courses, 

 though not at the head), 32 in number, and 11 shallow pools at the 

 same general elevation, all of them at or above timber line. Their 

 general characteristics have already been pointed out. 



The fauna of these lakes, while less abundant in species and indi- 

 viduals than that of the lower lakes, is by no means meager and in- 

 cludes 17 species, some of which were found in certain lakes m con- 

 siderable abundance. The fauna of this zone is characterized by (1 ) 

 the greater abundance here of certain species than in other zones, and 

 (2) the absence of a considerable number of species that are present 

 in lower zones. 



