176 MEMOIRS OF THE CARNEGIE MUSEUM. 



Delaware at Shawnee and at Kunkletown (in a stream tributary to the Lehigh 

 River) I found it to the northwest of the Blue Mountain (Kittatinny Mountain). 



No trace of it has ever been seen in the Susquehanna and Potomac drainages, 

 but it is positively present in the Rappahannock-drainage in Virginia, and here 

 again it is on the Piedmont Plateau. (Ortmann, 1913a, p. 319.) 



According to Simpson (1900), it goes from "northern New England to Vir- 

 ginia," but even in New England and New York locality-records are quite scarce. 

 It is known from the Connecticut River, Hartland, Windsor Co., Vermont (Mar- 

 shall, 1895), from Massachusetts (Marshall, but not mentioned by Gould-Binney, 

 1870), and in Connecticut from the Housatonic River (Linsley, 1845) and Mixville 

 (Cheshire) New Haven Co. (Marshall) (see also Johnson, 1915). In New York, 

 it is known from the southeastern portion (Marshall, without exact localities). 

 Otherwise definite records are missing, but the extension of its range southward to 

 Virginia, as given by Simpson, has been confirmed by the present writer with regard 

 to the Rappahannock River. 



For the present it may be said that the chief feature of the distribution of 

 this species is its erratic character. As far as I can now see, it seems to be most 

 likely a species of the Piedmont Plateau. Its apparent absence in the Susquehanna 

 and Potomac drainages may be due to the incomplete investigation of the Piedmont 

 tributaries of these rivers. 



At Kunkletown and in the Rappahannock-drainage, I found this species in 

 very small streams ("runs"), in strongly flowing water, and in rather coarse 

 gravel, but at each locality only few specimens. In the Delaware at Shawnee it 

 was (one specimen) in a small branch of the river in sand and moderate current. 

 In the only case where I collected large numbers (about fifty), I found it under 

 very different conditions, and that was in the canal at Manayunk at a time when 

 the water had been drained off and only a few inches of water were left in the 

 middle. Here the bottom consisted of larger and smaller stones, the interstices 

 filled with sandy mud. This, however, cannot be regarded as normal, and the 

 ecological conditions of this species remain to be studied. 



Subgenus Alasmidonta Simpson (1900). 

 Alasmidonta Simpson, 1914, p. 493, -1- Bullella, ibid., p. 508; Alasmidonta Ort- 

 mann, 1914, p. 45. 



Type Monodonta undulata Say. 

 Only one species in Pennsylvania belongs to this subgenus. 



