REPRODUCTION IN THE UNIONID^ 
87 
Tetragense with all four gills functioning for this purpose. Bas- 
ing his supposition largely upon shell characters and facts of geo- 
graphical distribution, he further concludes that the Homogense 
marked the next step in marsupial differentiation, while the Heter- 
erogense and all other groups in which a portion of the outer gills 
only is structurally modified for receiving the eggs are the latest 
product of the evolution of the Unionidse. That this series cor- 
rectly represents the phylogenetic sequence in the appearance of 
the marsupial modifications would seem to be borne out by the 
structural conditions existing in the several types, so far as we 
have examined them. 
BREEDING SEASONS 
In connection with our study of artificial propagation ot fresh- 
water mussels, we have found it necessary to collect data bearing 
upon the breeding seasons of a fairly wide range of species, since 
the records of previous observers, for North American Unionidse 
at least, have been insufficient to enable us to determine the full 
extent of the seasons, especially in the case of some of the more 
important commercial species. Although our observations have 
been largely confined to species occurring in the upper Mississippi 
Valley and have been concerned primarily with species of com- 
mercial value, we have continuous records throughout the entire 
year for a number of important genera, and in every case the exact 
stage of development of the embryos has been determined by 
microscopic examination. Many thousands of such observations 
have been made, so that we are now in possession of detailed 
information dealing with the duration and progress of the periods 
of gravidity obtaining in over a dozen genera of the Unionidae. 
We have confirmed the conclusion first reached by Sterki('95) 
that the North American Unionidse, with respect to their breeding 
seasons, fall into two classes, the so-called '^summer breeders " and 
winter breeders." The latter designation, however, is not 
strictly appropriate, for in the species which belong to this group, 
the eggs are fertilized during the latter part of the summer, usually 
in August, and the glochidia, which are carried in a fully developed 
