NO. 2226. DISTRIBUTION OF ENTOMOSTRACA—DODDS. 85 



spring and attain a higher temperature during the summer, so that 

 for temperature conditions and faunal characters the smaller lakes 

 at higher elevations resemble the larger ones at lower altitudes. As 

 a result of this condition, faunal zones can not be drawn definitely 

 on the basis of altitudinal limits, but there is overlapping due to the 

 size difference of lakes. In lakes of the Colorado Rockies I find the 

 temperature difference above referred to, but have been able only 

 indefinitely to correlate it with faunal differences. It is not easy to 

 compare zone for zone with the Colorado lakes, but it is probable that 

 his birch zone agrees with the upper part of my montane zone, and 

 that his other two correspond to my alpine zone, although I have 

 not encountered conditions as severe as those of his highest lakes. 



PLACE OF COLORADO FAUNA IN WORLD DISTRIBUTION. 



Wesenberg-Lund (1908), in summarizing the present knowledge 

 of the fresh-water plankton of the earth, classifies lakes under the 

 following zones : 



(1) Arctic lakes : Those in Arctic America, Greenland, Franz Jos- 

 eph Land, Spitzbergen, Nova Zembla, and Arctic Siberia. 



(2) North European lakes : Scotland, Iceland, Norway, Sweden, 

 Finland, etc. 



(3) Central European lakes of the level country. 



(4) The Mediterranean lakes. 



(5) Tropical lakes : Those of Central Africa and places of similar 

 climate. 



To these zones he adds (6) the Central European alpine lakes. 



He describes the physical and climatic characters in each zone, and 

 so far as possible gives the constitution of the plankton. 



Tliough a detailed comparison can not be made, it is possible to 

 assign some elements of our Colorado fauna to certain of Wesenberg- 

 Lund's zones. The lakes of the alpine zone, though less extreme 

 than the northernmost of those, belong very close to the arctic lakes. 

 Though there are few common species, the general similarity of the 

 fauna is evident. Holo'pedium gihherum,, which he considers as very 

 nearly confined to the arctic regions, is found in our alpine and mon- 

 tane zones. Other species considered as arctic by him are DapJinia 

 longispina, Bosmina longirostris , CeriodapJinia pulchella, and Ohydorus 

 sphaericus. All these range southward from the arctic regions. In 

 Colorado they are not by any means the most important nor the 

 highest of our alpine forms, but they do range high in the mountain 

 region. The importance of Diaptomus in both faunas is marked, 

 though it is not surprising that there are no common species of this 

 genus. The essential agreement of the climate of our alpine zone 

 to that of the zone occupied by the arctic lakes has been pointed out 

 in the section on climate. 



