ORTMANN: monograph of the NAIADKS of PENNSYLVANIA. 25 



into this region from Lake Eric near Buffalo, and that it is the form parvula which 

 originally entered the tributaries and canal, but which under the changed environ- 

 ment, again assumed the shape and the characters of the form of the creeks, called 

 flava. This is a very interesting phenomenon, and appears to indicate the very 

 important influence of environment upon the shape of the external characters of 

 the shell. If this were the only instance of the kind, we might regard it with 

 suspicion, but we shall become acquainted with other facts of a similar nature, and 

 in order not to loose sight of this instance, it is emphasized here. 



Finally attention should be directed to the great tendency to develop local 

 forms within the flava-gvonp. Although we have distinguished onh' four main 

 forms {undata, trigona, flava, parvula) some of these include several local types. 

 For instance, the flava of the AUeghenj^ River and Crooked Creek in Pennsylvania 

 is distinguished by small size, while in the creeks of southwestern Pennsylvania 

 a much larger form prevails. In the Little Kanawha River in West Virginia is a 

 very peculiar race of flava, distinguished by a shape, which is considerably drawn 

 out at the lower posterior end. A peculiar small race of trigona is found in Elk 

 River, West Virginia, and the forms from Arkansas also have certain charac- 

 teristics of their own. These conditions are very interesting, but can be studied 

 only with the helj) of larger series from the difYerent localities. 



Genus Amblema Rafinesque (1820). 

 Crenodonta Ortmann, 1912, p. 245; Crenodonta (Schlueter) (section of Quadrula) 

 Simpson, 1914, p. 813; Amblema Frierson, 1914a, p. 7; Amblema Utter- 

 back, 1916, J). 31. 



Type Amblema costata Rafinesque. 



Only one species, ^4. plicata, is found in Pennsylvania, within which, however, 

 two well-marked races may be distinguished. 



Key to the forms of A. plicata. 

 oj. Shell smaller, generally more swollen, less elevated posteriorly. Sculpture loss developed, chiefly 



so upon the posterior slope .1 . plicata. 



flj. Shell larger, generally more compressed, and more elevated jiosteriorh-. Sculpture better developed, 



chiefly so upon the posterior slope A. plicata costata. 



• Amblema plicata (Say) (1817).-" 



Quadrula plicata hippopcea (Lea) Simpson, 1914, p. 816; Amblema plicata (Say) 

 Utterback, 1916, p. 33. 



^" Not 1816, as given by Simpson. (See Binney, Bibliography, 2, Smithsonian Miscell. Coll., 9, 

 1869, p. 277.) 



