LYMNZIDA OF NORTH AMERICA. 23 
“The originally dry plains of the eastern part of the state are 
almost unbroken up to their very abrupt meeting with the foothills 
of the Front Range, at least from Colorado Springs to the northern 
boundary of the state. Through the plains run occasional streams, 
along which were originally some cottonwoods, willows, etc., but, 
generally speaking, except close to the borders of the streams, the 
country is semi-arid, with occasional shallow lagoons or lakes, often 
of considerable area but seldom of any depth, many of them being 
dry through a large part of the year, but filling up during the spring 
rains and run-off from the snows. These lakes are believed to be 
largely wind-formed, though that is not at all certain in many cases. 
At any rate, they are not of glacial origin, as the glaciers in this 
region did not extend below the 7000-8000 foot line, while the plains 
meet the foothills at from 5000 to 6000 feet. 
“Wherever the water from the streams can be economically 
carried in open ditches, by going upstream far enough to get sufficient 
head, a great change has been wrought in the character of the country 
in the last forty years, much of it within the last twenty. Many of 
the lakes have been enlarged and deepened by the building up of their 
dams, other reservoirs have been formed, dotting the plains everywhere 
within convenient reach of the streams, and ditches extending in all 
directions. Seepage water from ditches and reservoirs form many 
smaller pools, which are apt to be dry except during the irrigation 
season, just as the numerous pools formerly formed by overflow water 
along the streams during the spring run-off dry up later in the season. 
Many of the lakes are choked with vegetation usually found in such 
situations, while others are quité free therefrom. The bottom is apt 
to be of fine mud. In the mountains above 8000 feet there are nu- 
merous lakes, the origin of some of the lower ones being in doubt, 
but all the higher lakes being of glacial origin. The lower lakes, the 
only ones in which I have found Lymnzeas, are usually very shallow 
lagoons, choked up by vegetation, including sedges, etc., and nearly 
or quite drying up in late summer.” 
Dr. Henry A. Gleason has recently? made an interesting ecological 
survey of Isle Royale, Michigan, and his remarks on the ecological 
relations of the Lymnzas of this area are of great value. He says: 
“Tonkin Bay is a small inlet about a half a kilometer long, opening 
to the east upon the lake, and with steep, approximately parallel sides. 
It is narrowed half way up by two beaches lying perpendicular to its 
length. By this the wave action on the upper part is reduced, but 
1An. Rep. Ecol. Surv. Mich., 1908, pp. 60-62. 
