66 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.54. 



months. These studies of maxima and minima show more clearly 

 than does the annual or monthly mean the rigorous nature of the 

 climate and short duration of the summer season in the region of the 

 highest lakes. 



The above data regarding climate do not touch directly on the 

 medium inhabited by the Crustacea — the water. To an aquatic 

 animal climate means water temperature, not air temperature, and 

 the data just given are of importance only because the temperature 

 of the water is determined by that of the air and by general climatic 

 conditions. Nevertheless it is desirable that data be given concern- 

 ing the temperature of the water during the summer, the length of 

 time free from ice, etc. Such records will be presented as a part of 

 the account of the lakes themselves. 



TOPOGRAPHY AND DESCRIPTION OF LAKES. 



The eastern portion of Colorado lies in the region of the Great Plains, 

 with an elevation of from 4,000 to 5,000 or 6,000 feet. The plains 

 have a gradual slope toward the east, the valleys are broad, and 

 the hills gently rolling. In this region natural bodies of water are 

 few and small, being limited almost entirely to transient pools and 

 ponds which are dry for a considerable part of the year. In addi- 

 tion to these natural ponds there are many pools, ponds, and reser- 

 voirs which owe their existence to irrigation and are filled periodi- 

 cally from ditches. The largest of these are reservoirs a mile or 

 two in the largest dimension, containing water throughout the year, 

 but subject to great fluctuation in level. Another quite frequent 

 type includes the cattle ponds, depressions 2 to 4 feet deep and 50 

 to 100 feet across, scooped out to hold water for stock. The water 

 is commonly muddy from the clay bottom, is frequently very foul 

 with the droppings of the stock which water there, and seldom con- 

 tains much plant growth of any kind. There are long periods with 

 neither outflow nor inflow and they may be entirely dry for con- 

 siderable periods. Some of these have a very rich fauna. 



In drawing conclusions about distribution it must be borne in mind 

 that these artificial bodies of water are of recent origin, and it is en- 

 tirely probable that their development has been more rapid than the 

 migration of plankton Crustacea, so that an equilibrium has prob- 

 ably not been reached. This condition may account for the absence 

 of certain species from the plains which might be expected there. 

 The cHmatic conditions of these lakes present no facts of great inter- 

 est, being essentially hke similar bodies of water in other parts of 

 the Mississippi Valley. During the winter months, from the last of 

 November till the close of February, they may be covered with ice, 

 and the water temperature only a little above the freezing point. 

 From May to September the temperature during the day commonly 



