278 
BULLETIN OF THE UNITED STATES FISH COMMISSION. 
After several washings Ihe collections are preserved in 3 to 6 per cent formalin 
(3 to G parts commercial formalin to 100 parts water). They can not be preserved 
in alcohol without causing extensive shrinkage, rendering them useless for further 
study. 
These collections may later be examined under a lens in order to study the 
rotifers belonging to any family, genus, or species, and the specimens desired picked 
out by means of a pipette drawn to a capillary point. The different species are 
sorted into different watch glasses, and the blackening due to the osmic acid is 
removed by drawing off most of the formalin and adding a few drops of hydrogen 
peroxide for a few minutes. As soon as the desired degree of bleaching is reached 
the hydrogen peroxide is replaced by formalin. The formalin should be changed 
several times and allowed to stand several hours before mounting the specimens, 
otherwise bubbles of oxygen may appear under the cover glass after it is sealed. 
Specimens which have not been in osmic acid long enough to require bleaching 
are better in some respects than those that have been bleached by the hydrogen 
peroxide, as the latter removes the pigment from the eye, as well as the blackening 
due to the osmic acid. 
The specimens are then mounted in hollow-ground slides. The slides should 
be thin and the concavities shallow, so that it ivill be possible to use high powers of 
the microscope. The specimens are transferred to the concavities along with some 
of the formalin, and covered with a circular cover glass. It is best not to leave any 
bubbles of air beneath the cover. The superfluous fluid is withdrawn from the 
edge of the cover with a bit of filter paper, and the cover is then sealed. 
It is, of course, necessary to use some sealing material that will not allow water 
to evaporate through it. Mr. Rousselet, the originator of this method of mounting 
rotifers, recommends the following for sealing the mounts: After fixing the cover 
with a ring composed of a mixture of two-thirds gum damar with one-third gold 
size, there are added two coats of pure shellac, followed by three or four coats of 
gold size, allowing twenty-four hours for each coat to dry. 
The following account of the liattulidce is based on the study of 101 collections, 
made as above, and representing about half as many different stations. These 
collections were mostly made about the shores of Lake Erie, during the summers of 
1S9S, 1899 and 1901, while the writer was connected with the biological work on the 
Great Lakes carried on by the United States Fish Commission. The following- 
regions were examined with special thoroughness: 
1. The region about the islands in the western part of Lake Erie. 
2. The south or Ohio shore of Lake Erie, in the region known as East Harbor, 
some distance from Sandusky, Ohio. 
3. The lake shore and river at Huron, Ohio. 
4. The region about Erie Harbor, Pennsylvania, including the swamps and 
ponds on Presque Isle. 
5. Long Point, on the Canadian shore of Lake Erie. 
6. Many collections have also been made about Ann Arbor, Mich., in the Huron 
River, and in a number of small streams and ponds in the neighborhood. 
These collections have been supplemented by specimens and notes furnished 
by a number of investigators in Europe, as mentioned in the introduction. 
