Muttkowski—The Fauna of Lake Mendota, 
383 
the sand which serves to smooth the shore. The rachion re¬ 
sembles the gravel shore with its fluctuating rubble. It is 
composed of smooth stones and marks the point where the in¬ 
teraction of waves and undertow is most violent. In shoals 
the rachion is entirely detached from the shore proper, some¬ 
times a hundred or more meters distant. Furthermore, in 
such shoals there are '‘secondary’’ breaker lines found toward 
the shore, usually a short distance behind the first. The two 
rachia are comparable to the "outer” and "inner” bars of the 
ocean shore. 
The lower limit of wave action is marked by the shell zone. 
Sand and other heavier particles carried by the Avaves are soon 
deposited, while the lighter shell fragments are carried farther 
down the slope to the limit of wave action. In Lake Mendo¬ 
ta this lies immediately below the plant zone. 
Winds and waves dominate the complex and the upper dis¬ 
tribution of the littoral fauna and flora. Wave action deter¬ 
mines the floral complex in that it banishes all emergent and 
floating vegetation from the lake proper (by "floating,” 
plankton flora is not meant); it controls the upper distribu¬ 
tion in that it banishes all plants from the shore area and the 
rachion, except the filamentous alga Cladophora. 
Among the fauna the determining influence of molar agents 
is equally potent. Wave action controls the complex in that 
it banishes all surface breathers (with a feAv exceptions); it 
limits the distribution of water-breathers in that it permits 
only dingers and burroAvers on the shore and in the rachion. 
Waves furthermore aid the circulation of the lake, and thru 
the roiling of the bottom affect the clearness of the water, and 
hence the penetration of light for photosynthesis. 
Temperature and Circulation. —In early spring after the ice 
goes out the Avater absorbs heat; thru the influence of the wind 
and convection currents the water is set into complete circula¬ 
tion so that the heat is distributed thruout the entire depth of 
the lake. As the days grow warmer, the surface Avater absorbs 
heat faster than can be communicated to the lower depths of 
the lake. There is a concomitant change in the quantity of the 
dissolved gases. This difference in dissolved gas and circu¬ 
lation increases as the summer progresses, and a zonation is 
established in the lake: (a) an upper, well oxygenated, Avarm 
layer, the epilimnion, comparable to a zone of photosynthesis; 
