REPRODUCTION AND ARTIFICIAL PROPAGATION OF FRESH-WATER MUSSELS. 1 43 



Symphynota. — Embryos during August ; ripe glochidia from late September to late 

 June. S. complanaia is a species which we have had on hand constantly for several 

 years, and we have followed it continuously through the year. Spawning is most active 

 in June. 



SHORT PERIOD OF GRAVIDITY. 



In the species having the short period of gravidity the entire breeding season is 

 confined to about four months, as it extends only from about the end of April to the 

 middle of August, and the glochidia are discharged as soon as they are fully developed. 

 It is highly probable, however, that the beginning of the breeding season is influenced 

 to a certain extent by temperature, for it would seem that ovulation may be postponed 

 for some weeks by cold weather at this time of the year. It was first pointed out by 

 Sterki (1895) that these summer-breeding forms are confined to a limited group of 

 genera, and Ortmann (igi i) has emphasized the fact that it is only the genera having the 

 least specialized marsupia that possess this apparently more primitive breeding season; 

 these are the genera which constitute his subfamily Unioninae. Margaritana, unques- 

 tionably a primitive form, likewise breeds only in the summer. In all of these genera 

 the structure of the marsupium approaches most closely that of the respiratory gills; 

 none of the special modifications, so prominent a feature of the marsupium of other 

 genera, is present. There is apparently, however, one exception, for, as will be shown 

 below, our records indicate clearly that Obliquaria, which has a highly specialized marsu- 

 pium, is a summer breeder. 



The following are the species which we have obser\'ed to have the restricted breeding 

 season : 



Obliquaria reflexa. Quadrula pHcata. 



Pleurobema aesopus. Quadrula pustulosa. 



Quadrula ebena. Quadrula trigona. 



Quadrula heros. Quadrula (Tritogonia) tuberculata. 



Quadrula lachrymosa. Quadrula undulata. 



Quadrula metanevTa. Unio complanatus. 



Quadrula obliqua. Unio gibbosus. 



The following species, which do not appear above, have been determined by Ort- 

 mann (1909) to be summer breeders: Unio crassidens; Pleurobema clava and coccinea; 

 Quadrula kirtlandiaiia, rubiginosa, and subrotunda. Our list, on the other hand, sup- 

 plements his by the addition of several species of Quadrula, for which data have pre- 

 viously been either entirely wanting or quite meager. 



Obliquaria. — Since all of the forms which carry the glochidia over the winter have a 

 highly specialized marsupium, we should expect that Obliquaria, whose marsupium is 

 of such a nature, would also have the long gravid period. This expectation would be 

 further strengthened by the fact that the very closely related genus Cyprogcnia belongs 

 in the former group, as has been seen. It was therefore with some surprise that we 

 found 0. reflexa breeding during the summer. Our record is as follows : Embryos from 



