42 PROCEEDINGS OF THE NATIONAL MUSEUM VOL. 125 
ing the same streams and presumably the same hosts. A cedrus-like 
stock that gave rise to P. distichus to the north and a more primitive 
member, P. simondsi, to the south, moved by way of former con- 
nections with the Cumberland into the New River basin to produce 
P. alcicornus. 
The history of the genus Pterodrilus is conceived in broad outline, 
then, to be like this: In early Miocene or pre-Miocene times a primi- 
tive stock of cambarincoloid branchiobdellids were epizoites carried 
by the progenitors of the modern crayfish fauna of the upland regions 
of eastern North America. These animals lived along the slopes of the 
present Appalachian uplift, represented today by the Cumberland 
Plateau and the Highland Rim, which was drained by a stream that 
corresponded to the present day Cumberland. From this center, early 
stocks moved into the Ozarks and perhaps on into Mexico with the 
progenitors of the Mexicanus Section of the crayfish genus Procambarus 
(Hobbs, 1967, pp. 13-15) and produced the species P. missouriensis 
and P. mexicanus. Pterodrilus choritonamus and P. cedrus are the 
survivors (and representatives of the two lineages produced) of this 
early diversification that remained in the area of their origin and P. 
hobbsi is a more advanced member of the choritonamus-mexicanus 
lineage that has not only remained in the Cumberland basin but has 
successfully invaded the Tennessee system and more recently the 
New River drainage. Some of the early members of the missouriensis- 
cedrus lineage have also moved eastward, with one and possibly two 
(P. simondsi and the unnamed animals) remaining today in the 
Hiwassee basin as relicts. This lineage also gave rise to the advanced 
species, P. alcicornus, in the New River basin. Moving to the north, 
most likely by way of the Kentucky River or a nearby stream, another 
branch of this lineage gave rise to P. distichus, which remained in 
the Kentucky region throughout the Pleistocene, and in Recent 
times has followed its crayfish hosts (primarily species of Orconectes) 
into the Great Lakes and the St. Lawrence basins. 
These migrations have left three regions in which primitive species 
remain today: the original home, the Cumberland basin; the Ozarks 
in the Missouri and Arkansas river systems (and possibly the eastern 
slopes of the Sierra Madre Oriental in Veracruz); and the Hiwassee 
basin along the southwestern slope of the Blue Ridge. Of the dominant 
members of the genus, P. hobbsi is a product of the original diversi- 
fication in the Cumberland that today is most successful in the 
Tennessee basin; while of a second radiation of the P. cedrus lineage, 
P. distichus has invaded the recently glaciated areas to the north, and 
P. alcicornus has made its principal home in the valley of the New 
River. 
