232 Proceedings of Indiana Academy of Science 
Needham and Lloyd’s Figure 191 is likely to mislead one to regard 
the 10-20 foot zone as the most productive in Winona Lake. 
River mussels, as well as lake forms, have preferred types of bot- 
tom, hence “shoals.” 
Having in the main corroborated Headlee and Simonton’s analysis, 
with respect to the three factors given above, and compared the present 
situation with that of fifteen years ago, let us consider some of the fac- 
tors ignored or rejected by them. 
(4) Sex can be eliminated. Both males and females seem to occur 
over the entire range. 
(5) Age is probably pertinent. First because juveniles are rarely 
collected on the grounds where adults are most abundant. In the second 
place, while adults are prevalent on certain types of bottom, some of 
them must have migrated thither, for the host fishes of course do not 
drop the young mussels upon selected bottom. It will be interesting to 
learn the evolution of the parasitic habit of the bivalves, the origin 
of specific infection, and the correspondence in preferred habitat between 
given mussels and their respective hosts. The matter of age is very 
uncertain due to the rare finding of juveniles. 
(6) Pressure has been shown (in paper No. II of this series) to 
initiate and to govern the movements of mussels. Their distribution is 
partly due to this factor. It is probably of greater importance inshore 
than in deep water. The pressure difference within a few inches of 
the surface must be greater than differences of several feet in deep 
water, at the outer limit of the range. Physiologically the change from 
_ twelve inches to six inches should be the equivalent to a change from 
twelve feet to six feet. The most active movements of mussels actually 
take place in water of slight depth. 
(7) Light has a directive influence upon the movements which may 
sometimes affect distribution. The experimental demonstration is dis- 
cussed in the above named paper. 
(8) Relation to the Epilimnion. The above factors without further 
additions are sufficient to account for the adjustment to favorable en- 
vironment. Yet it is at least a happy coincidence that the most suitable 
bottom, depth, etc., occur in the epilimnion. Otherwise mussel life would 
have been impossible. Food supply, temperature, and oxygen are at the 
optimum where the bottom is also most favorable. Furthermore these 
conditions are best fulfilled during the summer months, the time of 
highest metabolic activity of the animals. 
The thermocline of Winona Lake begins at a depth of fifteen to 
twenty feet. Therefore the contours which represent its contact with 
the lake bottom are quite near the boundary of the wave-cut terrace. 
The terrace is thus washed by the epilimnion only, and the hypolimnion 
lies wholly outside the terrace. Conversely, the greater part of the lake 
bottom lies beneath the hypolimnion. These facts are of importance to 
the mussels in the ways mentioned above. 
(a) Temperature. The development of a thermocline due to the 
thermal resistance of water to mixture results in the maintenance 
throughout summer of low temperatures below its level. Instead of a 
distribution of the heat of summer throughout the water, the epilimnion 
