THE NAUTILUS. 91 



THE BREEDING SEASON OF UNIONIDJE IN PENNSYLVANIA. 



BY DR. A. K. ORT.MANN. 



Since C. T. Simpson published his new system of the Unionidx 

 (Proc. U. S. Nat. Mus. 22, 1900), which is founded, in a large part, 

 upon the " marsupium " of the female, this latter organ must be re- 

 garded as one of the most important features of the soft parts of the 

 mussels, and should be known in every species. Yet there is a large 

 number of species, in which it has never been seen by anybody. 

 That certain species are very rarely found with the marsupium de- 

 veloped, is, in my opinion, chiefly due to the fact that they are not 

 easily obtained at the period when they are gravid. As Sterki has 

 first suggested (NAUTILUS, 9, 1895, p. 91), there are two groups 

 among our mussels with regard to the period of gravidity l ) : summer 

 breeders and winter breeders. In the summer breeders the short 

 "breeding season " falls into the early summer months (May, June, 

 July) ; now since many of the species which belong or may belong 

 here are characteristic for the larger rivers, avoiding smaller streams, 

 and since just at this time the rivers very generally are swollen and 

 muddy by copious summer-rains, it is practically impossible to col- 

 lect them in the gravid condition. 



During the summer of 1908 we had exceptionally dry weather in 

 our region ; the stage of the rivers in Pennsylvania was already in . 

 the beginning of July very low, and thus the writer succeeded in 

 finding several species in a gravid condition, which generally at that 

 season are out of reach. In addition, he has collected other species 

 in the smaller streams at various seasons, and has found gravid 

 females, both of summer and winter breeders. The following is a 

 list of them, which also intends to give the previous records for those 

 species which are found in Pennsylvania:* 



1 In order to avoid misunderstanding, I want to state expressly that by 

 " period of gravidity " or '' breeding season " I mean the time when the gills, 

 or part of the gills, which forms the " marsupium," are filled with eggs or 

 embryos. 



2 See : Lea, I ; Observations, II, 1838, p. 51 ff.; Ill, 1842, p. 231 ; VII, 1860, 

 p. 221 ; X, 1863, p. 412, etc.; Sterki. V, NAUTILUS, 9, 1895, p. 91 ; 12, 1898, p. 

 18; Amer. Natural., 37, 1903, p. 103; Baker, F. C., Bull. Chicago Ac., 3, 

 1898 (passim) ; Conner, C. H., NAUTILUS, 21, 1907, p. 87. 



