108 University of California Publications in Zoology (Vou. 9 
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It will be seen that by introducing the electrical connections 
it is possible to have each cage, or unit for the accommodation of 
one animal (mouse or rat), independent of the others and also 
to place it in any desired position with regard to the chrono- 
graph. Three cages have been arranged to make their records 
on one chronograph drum. A single cage and its connections 
with the chronograph drum are shown in plate 15. 
The following description applies to each cage or unit. The 
wire sides and top of the cage are supported on two uprights 
(up., pl. 14, fig. 4), and are independent of its floor. The floor 
consists of two parts, each made of sheet zinc, a smaller central 
and a larger marginal floor. They are shown at the right in 
plate 13, figure 2. The larger floor (m. fl., fig. A, top surface ex- 
posed in pl. 13, fig. 2) has the center cut out to accommodate the 
smaller; at the edges the zine is turned up, as clearly shown in 
figure 2. The smaller floor (under-surface exposed in fig. 2) also 
has the edges turned up, but to a less extent. The smaller part, 
which carries the nest, fits loosely within the opening of the 
larger one. These two floors instead of being suspended from 
levers above, as in the earlier model, are supported from below 
on levers of special construction (figs. A and B and pl. 13, fig. 2). 
Each lever consists of a transverse bar (fr. br.) at right angles 
to which are attached three parallel bars, one (s. br.) project- 
ing beyond the cage, the other two (p. br., p. br.) terminating 
under the middle of the cage. The two levers are so constructed 
that the smaller, carrying the nest floor, occupies a position 
inside the larger (see pl. 13, fig. 2). The fulerum of each lever 
is formed of two slender wire nails (ful. nl. and ful. nl.), which 
fit into slots in the sheet-metal lever supports, as clearly shown 
at lr. sup., figure A and plates 14 and 15. The unpaired bar 
(s. br.) of each lever carries a counterweight (wt.); the paired 
bars support the appropriate floor. On the under side of each 
of the floors are two metal supports (fig. A, fl. sup.) similar to 
those on which the levers rest (see right hand of fig. 2, pl. 13, 
also pl. 14). In each support is a slot, which rides on a slender 
wire nail (nl., fig. B) driven into the side of each of the paired 
bars near their free ends. Each floor thus rests on the ends of 
the paired bars of its lever. The floors are prevented from 
