NO. 3668 PTERODRILUS—HOLT 4] 
to connect the parts of its range that now appear to be disjunct. 
If so, the upper reaches of the Cumberland in Kentucky remain the 
likely site of origin for P. hobbsi. The few scattered records from the 
Big Sandy and the New Rivers indicate that the species is still ac- 
tively extending its range, and much of the spreading of P. hobbsi 
may well have occurred quite recently. 
The migrations of P. distichus, P. hobbsi, and P. alcicornus may 
have occurred rather recently, but the movement of the stock that 
gave rise to P. simondsi must be older. Although it is believed that a 
common ancestor gave rise to both P. simondsi and P. alcicornus, the 
former clearly stands at a lower level of evolutionary advance as is 
indicated by the primitive nature of its reproductive systems. It is 
known only from the tributaries to the Ocoee River and one locality 
in the Nottely, both parts of the Hiwassee River system of the Ten- 
nessee River basin. Pterodrilus simondsi is found, then, at the south- 
eastern periphery of the range of the genus in an isolated part of the 
somewhat isolated Hiwassee basin. Its ancestors came from the 
Cumberland and its known distribution can be explained by postu- 
lating that the species was once widespread in the Tennessee basin 
but has been eliminated throughout all of its range except the small 
part in the Hiwassee by the more advanced, successful, and wide- 
spread species, P. hobbsi and P. alcicornus. In any case, though the 
origins of P. simondsi may not be as ancient as those of the relict 
species in middle Tennessee and the Ozarks, it is an older relative 
of P. alcicornus holding out in a relict status in a part of the Tennessee 
basin not yet successfully invaded by the latter. 
If the hodbsi-like animals mentioned above (p. 32) are conspecific 
with other populations of P. hobbsi, the Hiwassee drainage is being 
invaded by this more advanced species, but if, as seems more likely, 
these specimens represent a survival of the primitive stock that gave 
rise to P. cedrus, we have at the periphery of the present range of 
the genus a relict of the first radiation within Pterodrilus. 
Pterodrilus alcicornus is the most advanced and successful species 
of the genus. It is a native of the New River basin that has in recent 
times extended its range, probably by stream captures, into the James 
and Roanoke basins to the east, into the Big Sandy to the north, 
and, amazingly, into the Savannah in the south. The latter invasion 
can only have occurred by means of the streams of the Tennessee 
system that lie between the headwaters of the New and the Savan- 
nah in western North Carolina, a region that has been inadequately 
sampled. Still earlier, P. alcicornus had moved into the upper reaches 
of the Tennessee River system in southwestern Virginia and north- 
eastern Tennessee, where it is sympatric with P. hobbsi, often occupy- 
