ORTMANN: monograph of the naiades of PENNSYLVANIA. 



227 



not always, the shell is of a lighter color. When lighter, the color is greenish or 

 brownish olive upon the disk, and yellowish upon the posterior slope. 



L. H. D. Pr.ct. 



Size: (Males) 1. Wampum, Cat. No. 61.2842 67 mm. 57 mm. 33 mm. .49 



2. do. " " do. . 



3. do. " " do. . 

 (Females) 4. Wampum, Cat. No. 61.2842. 



5. Natrona, Cat. No. 61.4116. . 



60 



50 



47 



40 



6. Creekside, Cat. No. 61.3269 37 



..55 

 .50 

 .53 



.55 

 .51 



The lowest diameter ever observed is in a specimen from West Fork River in 

 West Virginia, where it is forty-six percent of the length. 



Sojt parts identical with those of 0. mbrotunda (See Ortmann, 1912, p. 328). 

 Glochidia figured by Ortmann, 1911, PI. 89, fig. 15. They measure: 0.20 X 0.23 

 mm. 



Breeding season: Glochidia have been foimd on Sept. 21, 1908, and again on 

 May 23, 1911; May 24, 1911; and May 27, 1908. On the latter date, discharge 

 has been observ^ed. This indicates, that the form is hradyticlic. 



Remarks: The essential difference between this and the normal form is the 

 compression of the valves. It is true that the color is also often lighter (as pointed 

 out by Lea) but this does not hold good for the average of the Pennsylvanian 

 specimens. As is evident by the measurements given, the diameter of the shell 

 is less than in 0. subrotunda; but it must be noted, that at certain places the two 

 forms are found associated, and that they actually and insensibly pass into each 

 other. The dividing line at the diameter of sixty percent is entirely artificial, 

 and does not express natural conditions. It has been introduced, as in similar 

 cases, simply for the sake of convenience. 



In the smaller creeks, var. levigata is found rather exclusivelj', while subrotunda 

 prevails in the Ohio. The transitional zone between them is precisely in the region 

 of Pittsburgh. Thus these forms must be regarded as varieties of one and the 

 same species. 



Localities in Pennsylvania represented in the Carnegie Museum: 



Ohio River, Neville Island, Allegheny Co. 



Beaver River, Wampum, Lawrence Co. (G. H. Clapp & H. H. Smith). 



Mahoning River, Mahoningto^oi, Lawrence Co. 



Shenango River, Harbor Bridge and Pulaski, Lawrence Co.; Clarksville, Mercer Co. 



Pymatuning Creek, Pymatuning Township, Mercer Co. 



Allegheny River, Natrona, Allegheny Co. ; Godfrej', Armstrong Co. 



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



