ORTMANN: monograph of the naiades of PENNSYLVANIA. 149 



and yellowish brown toward the lower margin (alternating according to the gTOwth- 

 periods). There is no, or very little, green present, and the rays upon the posterior 

 slope, if visible at all, are browniish black. According to my experience this form 

 is characteristic of the lagoons and beach-pools of Presque Isle, where it is quite 

 abundant, and very uniform in character. 



The other form is less elongated, higher and shorter, sometimes squarish, 

 with a very straight hinge-line, having a sharp anterior and posterior angle. Its 

 shell is generally more solid, and its color is distinctly greenish, often rather light 

 green, but passing into light brown, rarely into rusty. Its growth-rests are distinct 

 and rather regular, and the rays upon the posterior slope are dark green. This is 

 the form prevailing in Presque Isle Bay, chiefly on its north-shore, so called "Big 

 Bend." Some of these short, squarish forms resemble to a degree A. benedidensis 

 Lea (Obs. I, 1834, PL 16, fig. 48; and DeKay, 1843, PL 18, fig. 235), but I do not 

 think they should be called by this name,"" for there are intergrades with the 

 elongated form of the beach-pools. These intergrades are frequently found 

 associated with the normal (quadrate) form, and do not have the quadrate shape, 

 but are more or less subovate, much resembling the original figure of hea'sfootiana 

 (Obs. Ill, 1842, PL 20, fig. 44). The specimens collected in ponds at the mouth 

 of Elk Creek are all of this subovate form, with the typical footiana outline, and 

 at the extreme western end of Presque Isle Bay in quiet and rarely disturbed water 

 this ovate form is also present. In the lagoon at Waldamer Park I found speci- 

 mens, which are more elongated, and thus approach the beach-pool form, but they 

 have green (not rusty) epidermis. ' 



It is evident that the squarish, thick-shelled form of the surf-beaten northern 

 shore of Presque Isle Bay passes gradually into the elongated, thin-shelled form of 

 the lagoons and beach-pools; and that it should be regarded as an ecological 

 variation of .4. grandis footiana (with which it agrees in the inflated beaks), and 

 not as a form of benedictensis. 



The various forms of footiana are distinguished from ^4. cataracta by the 

 swollen and inflated beaks. Although cataracta has sometimes a rather inflated 

 shell, the beaks are always low, and not so prominent as in footiana. 



'"^ .4. benedictensis conies originally from Lake C'hamplain, and has been reported from other hikes 

 in New York. The Lake C'haiitauqua-form of A. grandis, mentioned above, might be regarded as a 

 dwarf form of it, and also the form from Lake Erie (Sterki, 1907fl, p. 394; Walker, 1913, p. 22). Mr. 

 Walker (according to a communication in a letter) considers the ([uadrate form and straight hinge-line 

 as the chief characteristics of his benedictcjisis, and admits that tiiis differs from the true benedictensis 

 in being more inflated. All this fits the present form. 



