ORTMANN: monograph of the naiades of PENNSYLVANIA. 205 



are rather small on the average. Farther up, and in the tributaries, it becomes 

 more and more abundant, and in all small creeks it is a common shell. 



The fact is remarkable, that S. edentulus is the only Naiad present in the 

 upper Youghiogheny. There is no other stream in Pennsjdvania, which offers a 

 parallel case, but such are found in several mountain streams in West ^^i^ginia. 

 This has been discussed elsewhere (Ortmann, 1913a, pp. 303, 305, 308; 357, 367 

 et seq.). 



Likewise on the Atlantic slope this species is scarce or missing in the large 

 rivers. This is most e\adent in the lower Susquehanna. It is present in the 

 Delaware near and above Trenton, New Jersey, but does not seem to be abundant. 

 It may be that in the estuary of the Delaware S. undulatus takes its place. 



We may therefore call S. edentulus a form characteristic of smaller streams. 

 Baker (1898a, p. 68) says indeed that it is found in the larger lakes and rivers on 

 muddj' bottoms, while Scammon (1906, p. 323) reports it to prefer mud and quiet 

 water, and to be most abundant in smaller streams (in Kansas). Its absence 

 in the larger rivers in Pennsylvania may be connected with its aversion to rough 

 bottom and strong currents. Even in the small streams it avoids riffles, but de- 

 lights in quiet and protected nooks, pools, and eddies, where there is a moderate 

 and rather uniform current, and a deposit of fine gravel, sand, or mud. It is often 

 found in the Z)iani/iera-patches. 



On the other hand this species goes into canals and lakes. Altogether, it 

 lives under a great variety of environmental conditions, and thus it is not aston- 

 ishing that it is so widely distributed. 



General distribution: Type locality, Wabash River (Say). 



According to Simpson (1900) S. edenttdus ranges over the "entire Mississippi- 

 drainage; the St. LawTence system and south in streams draining mto the Atlantic 

 to North Carolina; north in the British possessions to Lake Wimiipeg; south- 

 west to central Texas." (An Alabaman locality has also been recorded.) 



This is substantially correct, and this is possibly the widest range occupied 

 by any species of Naiad in North America. Its occurrence in Canada in the lower 

 St. Lawrence-drainage, as well as in the west, is well-established (Whiteaves, 

 1863; Bell, 1859; CaU, 1885; Dawson, 1875; Hanham, 1899; Christy, 1855; see 

 also our records from Ontario and North Dakota). Its presence on the Atlantic 

 slope from Maine (Lermond, 1909) southward is well kno-mi, and substantiated 

 by our records as far South as the Roanoke system in Virginia. It is said to go 

 to North Carolina (Simpson), but here the southern boundary is obscurely known. 



The center of radiation is apparently located in the JMississippi system (See 



