140 BULLETIN OF THE BUREAU OF FISHERIES. 



Although our observations have been largely confined to species occurring in the upper 

 Mississippi Valley and have been concerned primarily with species of commercial 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 fully confirmed the conclusion reached by Sterki (1895) that the North 

 American Unionidae, with respect to their breeding seasons, fall into two classes, the 

 so-called "summer breeders" and "winter breeders" — a distinction, however, which 

 had previously been pointed out by Schierholz (1888) for European forms and fre- 

 quently recorded by later observers. The designation "winter breeders," however, is 

 not strictly appropriate, for in the species which belong to this group the eggs are 

 fertilized during the latter half of the summer and the glochidia, which are carried in 

 a fully developed condition in the marsupium throughout the winter, are not discharged 

 until the following spring and summer. In the case of the summer breeders, the eggs 

 are fertilized during late spring and summer and spawning as a rule is over by the end 

 of August. 



In view of these facts, it would seem to accord better with the actual conditions 

 to separate the species with respect to the length of time that the glochidia remain in 

 the marsupium, designating them as those that have a "short period" and those with 

 a "long period" of gravidity, rather than to distinguish them as "summer breeders" 

 and "winter breeders," respectively, for with respect to the latter neither ovulation 

 nor discharge of the glochidia takes place in winter. This suggestion was made by us 

 in an earlier paper (1910b), and subsequently Ortmann (191 1) proposed the somewhat 

 awkward terms tachytictic and bradytictic (meaning quick-breeding and slow-breeding) 

 for Sterki's "summer breeders" and "winter breeders," respectively. 



The breeding seasons as here defined are based upon data collected in the middle 

 and northern sections of the United States, and in the absence of adequate records from 

 higher and lower latitudes, it is impossible to say to what extent a colder or warmer 

 climate might affect the period of gravidity That it would have some influence can 

 hardly be doubted, although a distinction between a long and a short season will prob- 

 ably be found to hold true in general. 



The breeding season is a generic character, for so far as our observations have gone 

 all of the species belonging to a given genus have essentially the same period of gravidity. 

 The prolonged period, furthermore, is correlated with the more pronounced structural 

 modifications of the marsupium which have been described above. 



