Muttkowski—The Fauna of Lake Mendota, 
389 
the north, where it passes thru the swamps; that region seems 
to present optimal conditions for Hydra. 
A few specimens of Hydra viridis Linne were taken on Chara 
at the mouth of Merrill Spring in August, 1914. This rare 
species has been obtained elsewhere only from spring-fed 
pools north of Lake Wingra. 
3. Bryozoa. 
Plumatella repens Linne. 
Like Spongilla, this species encrusts the under side of rocks 
along rocky shores and in the Yahara Spillway. Some dis¬ 
tance up the Yahara River I have found palmate growths of 
Plumatella on the underside of lily-pads; such growths are in¬ 
frequent in the lake on plants, altho they may be occasional 
on Potamogeton amplifolium. More minute growths of Bry¬ 
ozoa, belonging to the genus Predericella, and probably also 
to Lophopus, occur plentifully on the plants of the lake, and 
frequently on the bottom to a depth of eight meters. 
4. Platyhelminthes. (Table 5, p. 474.) 
Planaria maculata Leidy. 
This ubiquitous species is found practically everywhere in 
the lake. The color variations are considerable and one may 
be tempted to consider the extremes of specific distinction. 
It is possible, indeed probable, that several species have been 
lumped as P. maculata in the numerical estimates. A relative, 
P. doritocephala, is abundant in Merrill Spring; another spe¬ 
cies, P. agilis, is frequent in the springs and occasional in the 
lake. 
Polycelis nigra Ehrenberg. 
Dendrocoelium lacteum (Muller). 
These two species, so interesting because of their colors, ap¬ 
parently find conditions optimal in Merrill Spring, where they 
are thick on the stones in the channel. Both species are occa¬ 
sional in the lake, confined apparently to the rocky areas. 
Mesostoma ehrenbergii (Focke). 
This form was taken in four of about 350 catches. It is very 
common in Picnic Pond in May and June, but disappears as 
