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PREFACH. 
The exhaustion of the edition of ‘‘A Final Report on the Crustacea 
of Minnesota included in the Orders Cladocera and Copepoda’’ has 
led to a demand for republication. It has seemed unfortunate that 
the imperfections of this work, many of them due to the unsatisfactory 
state of the European literature, while others were the result of im- 
perfect information and faulty observation or hasty judgment, should 
be perpetuated. The original paper grew out of studies made chiefly 
while the author was an undergraduate at The University of Minne- 
sota and was very fully occupied otherwise, and circumstances have 
prevented him from anything but the most casual pursuit of the sub- 
ject since 1884. It might have appeared that the work had served its 
purpose, but there were many indications of a need for a comprehen- 
sive, synoptical survey of this field, and the present paper seemed the 
only approximation to sucha work. A _ serious illness, which has 
greatly impeded the progress of the revision at last decided on, has 
at the same time made it possible by forcibly relieving the author of 
more pressing duties. It would have been a great satisfaction to him 
to have given to the subject the thorough field work and comprehen- 
sive bibliographic elaboration it deserves, but strength and opportu- 
nity forbid. In default of this it might seem wiser to enumerate only 
American species, but experience has shown that geographical limi- 
tations do not prevail to any great extent in most of the families here 
considered. In such instances, Calanide for example, the attempt 
has been made to include all valid species and a full Synonomy. In 
the others as full a list is given as is now possible, and while this 
work may not be alone sufficient to enable the specialist to formulate 
new diagnoses and descriptions, it yet offers much fuller Synopsis 
than yet published and will give a bird’s-eye view of the taxonomy of 
the subject. 
The most important aspect of the subject, namely, the biological 
relations of these organisms, has been too generally neglected. {n no 
other group can the problems of parthenogenesis and heterogenesis 
be so easily and accurately studied. The transparency of the body 
