ORTMANN: monograph of the naiades of PENNSYLVANIA. 187 



imrtly in the Atlantic-drainage (Marshall and Baker, 18986), but these records 

 should be confirmed on account of the possibility, that this form may have been 

 confounded with the eastern forms.^^** Thus the northeastern boundary of A. 

 marginata is yet obscure. Northward this species has certainly crossed over into 

 the lake-drainage, as is shown bj^ a number of localities represented in the Carnegie 

 Museum, and has been reported for Ohio (Dean, 1890; Sterki, 1907a), Indiana 

 (CaU, 1896a and 1900), and Illinois (Baker, 1898«, 1906). In Michigan, it has 

 spread over the southern parts of the state, as far north as Roscommon Co. (Walker, 

 1898), and it is present in Ontario, north of Lake Erie (Carnegie IMuseum). 



In the Mississippi this species goes up from Iowa (Pratt, 1876; Witter, 1878) 

 to Minnesota (Grant, 1886; Holzinger, 1888). 



West of the Mississippi, it seems to be much rarer. It is in the Wapsipinicon 

 and Volga Rivers in Iowa (Geiser, 1910), and at Iowa City, Johnson Co., Iowa 

 (Marshall), in Missouri (Utterback, 1916), but not in Kansas (Scammon, 1906). 

 It is in the Ozark region of northern Arkansas (Carnegie Museum and Meek & 

 Clark, 1912) going to the Ouachita River in central Arkansas (Wheeler, 1918). 

 Thence southward it is missing. 



Alasmidonta (Decurambis) MARGINATA SUSQUEHANNA Ortmann. 

 Ortmann, 19136, p. 315 {et ■passim, sine descriptione) . 



Plate XII, fig. 4. 



Not previously recorded, except by Ortmann (19136). 



Characters of variettj: Shell somewhat smaller than that of the normal form. 

 Color of epidermis peculiar, brighter. Epidermis more or less brown (pale or 

 reddish brown), with distinct green rays, which have a very strong tendency to 

 break up into spots, which are very rarely absent. The posterior slope is always 

 light in color, pale brown, reddish or yellowish, with hardly any green rays, so 

 that it is in sharp contrast to the rest of the shell. 



The nacre is whitish, but has quite frequently salmon or pinkish tints, in fact, 

 delicate reddish tints are the rule in this variety, while the bluish-white of the 

 western marginata is rarely present. 



Finally the posterior truncation, although similar to that of many specimens 

 of the western form, does not exhibit the extreme development, which is so often 

 seen in the latter. 



"° Marshall, for instance, cites the Chemung and Tioga Rivers, belonging to the Susquehanna- 

 drainage. The Carnegie Museum possesses specimens from the Tioughnioga River, Susquehanna- 

 drainage, which surely are not typical marginata, but belong to the two eastern forms, var. susquehannce, 

 and the species varicosa. 



