620 BULLETIN OF THE BUREAU OF FISHERIES, 
of a large number in a state of nature, for under ordinary conditions it is 
extremely improbable that more than a very few glochidia would succeed in 
attaching themselves to a single fish. 
After a parasitic life of variable length, during which the metamorphosis 
is completed, the cyst breaks down, and the young mussel, thus liberated, falls 
to the bottom and leads thereafter an independent life. 
BREEDING SEASONS. 
We have confirmed the conclusion, first reached by Sterki (Nautilus, vol. 
IX, 1895), that the Unionide, 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 the brood chambers 
over the winter, are not discharged until the following spring and summer. In 
fact, in some cases the close of one breeding period may overlap on the begin- 
ning of the next, as one may still find in late July some females gravid with 
glochidia, while in others the eggs are passing into the gills for the next season. 
In the case of the summer breeders the eggs are fertilized during the late spring 
and summer, and the glochidia are in all probability discharged before the end 
of August, or certainly not later than the middle of September. In the species 
of this class of which we have the completest records, ovulation begins in May 
and early June and may last well into August, but after the end of August 
gravid females have not been found. 
In view of the above facts, it would seem in better accord with actual 
conditions to separate the species with respect to the length of time that the 
glochidia remain in the gills, designating them as those which have a “short 
period” of gravidity and those having a ‘‘long period,” rather than to dis- 
tinguish them as “summer breeders” and ‘‘ winter breeders,’’ respectively, for 
in the case of the latter neither ovulation nor discharge of the glochidia takes 
place in winter. The breeding season is undoubtedly a generic character, for, 
so far as our observations go, all species of a given genus belong to the same 
class. 
The following list shows the distribution, with respect to the period of 
gravidity, of the genera we have had under observation: 
Long period: Short period: 
Alasmidonta. Obliquaria. 
Anodonta. Pleurobema. 
Lampsilis. ‘ Quadrula. 
Obovaria. Tritigonia. 
Plagiola. Unio. 
Strophitus. 
Symphynota. 
