Feb., 1920] The Protozoa of Mirror Lake 91 



During this period, snow was on the ground almost con- 

 tinuously, and ice covered the lake. After February 9, coin- 

 cident with clearer weather, there were consistently higher 

 temperatures, and the snow and ice disappeared. 



DESCRIPTION OF MIRROR LAKE. 



"Mirror Lake" is a small artificial pond situated in a 

 natural ravine on the Ohio State University campus. Through 

 this ravine a creek formerly flowed from the east and emptied 

 into the Olentangy river nearly half a mile west of the present 

 lake site. In 1872 a sewer was built which drained the creek; 

 and about the same time the lake bed was excavated. The lake's 

 supply of water came from springs which had formerly fed the 

 creek. In 1895 all the water in the lake was emptied, most of 

 the animal and vegetable life was destroyed, and the lake bed 

 was enlarged by the addition of the small arm at its northwest 

 end. Since that time fishes, aquatic plants, and other forms of 

 life have been introduced into the pond. 



Mirror Lake is made up of two bodies of water, separated 

 only by a very narrow neck of land four feet wide (Plate I). 

 These will be referred to as the smaller pond and the larger 

 pond. A rapidly ascending grassy slope, covered with bushes 

 and trees, on the north side and a gently rising one on the south 

 side partly enclose the lake; the former is 25 feet high and the 

 latter 27 feet. West' of the lake the ground rises about nine 

 feet to the roadway, which is about 50 feet distant; while on 

 the east the rise is extremely gradual. 



The smaller pond is at most 50 feet wide (from north-east 

 to south-west) and 140 feet long (from north-west to south-east). 

 The springs on the north shore furnish it with a not very 

 copious supply of water; and the outlet is through a six inch 

 pipe at its north-west end. Only after a heavy rain does the 

 water in this pond reach a height sufficient to carry some of it 

 through this pipe into the larger pond. The average depth 

 in the smaller pond is about one and a half feet; the depth 

 in the deepest part does not exceed two and one-half feet. 

 The level of the water is usually about one foot higher than in 

 the larger pond. The bottom is covered with fine black 

 sediment and decaying leaves. More decay goes on in this 

 pond than in the other. Many tall white poplars {Populus 

 alba Linnaeus) shade this part of the lake from the sun, except 



