16 . MEMOIRS OF THE CARNEGIE MUSEUM. 



of the placentse a single one, in which they were pure white. This has remained 

 the only case of this kind. (I have seen over one hundred gravid females.) 



Breeding season: Gravid females have been observed on the following dates. 

 May 6, 1910; May 9, 1913; May 17, 1910; May 22, 1912; May 24, 1911; May 

 27, 1908; June 30, 1908; July 3, 1908; July 8, 1907; August 3, 1909; August 

 10, 1909. In the month of Maj' onl}^ eggs were found, at the other dates eggs and 

 glochidia, or only the latter. The species is typically tachytictic, breeding from 

 May to August. 



Remarks: This is a rather characteristic shell, but it has been frequently 

 confused with other species, chiefly with Pleurobema ohliquum coccineum (Conrad). 

 Nevertheless the peculiar, subtrapezoidal shape, the pale brown or reddish epi- 

 dermis, the flat sides, and distinct posterior ridge, always serve to distinguish it. 

 It is, however, quite variable, and, as we shall see below, three varieties may be 

 distinguished in Pennsylvania, and there are others outside of this state, which 

 have often been regarded as distinct species. In spite of this it has been positively 

 ascertained that these varieties actually intergrade in our region. 



Localities in Pennsylvania represented in the Carnegie Museum: 



Ohio River, Neville Island, Allegheny Co. 



Monongahehi River, Elizabeth, Allegheny Co. (D. A. Atkinson & Graf); Charleroi (G. A. Elirmann) 



and Mill vale, Washington Co. (J. A. Shafer). 

 Raccoon Creek, New Sheffield, Beaver Co. 



Chartiers Creek, Carnegie, Allegheny Co. (D. A. Atkinson & Graf). 

 North Fork Tenmile Creek, Amity, Wasliington Co.; South Fork Tenmile Creek, Waynesburg, Greene 



Co. 

 Dunkard Creek, Wiley and Mount Morris, Greene Co. 

 Allegheny River, Godfrey, Johnetta, and Kelly, Armstrong Co. 

 Crooked Creek, Rosston and Southbend, Armstrong Co.; Creekside, Indiana Co. 



Other localities represented in the Carnegie Museum: 



Lake-drainage: 



Genesee River, Chili and Rochester, Monroe Co., New York (R. H. Santens). 



Sandusky River, Fremont, Sandusky Co., and Upper Sandusky, Wyandot Co., Ohio (C. Goodrich). 



Miami-Erie Canal, Lucas Co., Ohio (C. Goodrich). 



Swan Creek, Toledo, Lucas Co., Ohio (C. Goodrich). 



Maumee River, Defiance, Defiance Co., Oliio (C. Goodrich); Fort Wayne, Allen Co., Indiana (C. Good- 

 rich). 



St. Mary's River, Rockford, Mercer Co., Ohio (C. Goodrich). 



Beaver Creek, Williams Co., Ohio (C. Goodrich). 



St. Joseph River, Indiana (B. Walker). 



Raisin River, Grape P. 0., Monroe Co., and Adrian, Lenawee Co., Michigan (C. Goodrich). 



Clinton River, Utica, Macomb Co., Michigan (B. Walker). 



