NO. 3668 PTERODRILUS—HOLT 39 
afford little insight into the problems of evolution and migrations of 
either the hosts or their epizoites: it is well established (Goodnight, 
1940, p. 65; Hobbs, Holt, and Walton, 1967, p. 75) that host specificity 
in the classical sense of a species-to-species correspondence does not 
occur. Yet it is worthy of note that the crayfish-branchiobdellid as- 
sociations as recorded under ‘‘Hosts’” for each species is consistent 
with the hypothesis that Pterodrilus originated in the Cumberland 
basin and spread from there with the ancestors of the hosts of today, 
mostly species of Orconectes. 
An attempt is made (fig. 12) to diagram more precisely the geo- 
P. distichus 
Great Lakes 
P baits, 
New River 
RP missouriensis 
P. distichus 
Kentucky River 
PR mexicanus 
R hobbsi 
Ozarks P choritonamus 
R alcicornus 
— Propterodrilid 
P- simondsi 
P cedrus 
Tennessee River 
Cumberland River 
Mexico 
Figure 12.—The evolution of the genus Pterodrilus. 
eraphical relationships of the species of Pterodrilus. The times at which 
all these migrations occurred cannot be determined on the basis of 
the evidence now available, but the original diversification of the 
early Pterodrilus stock took place well back in the Tertiary, and the 
movement of P. distichus into the glaciated regions of the north, of 
necessity, has happened since the last glaciation. Without attempting 
to pmpoint the events in time on the basis of the hypothesis developed, 
we can note that an early diversification of pro-pterodrilus stocks oc- 
curred in the Cumberland basin. Of this radiation, P. choritonamus and 
P. cedrus remain in the general area of their ancestral home as relict 
forms. A primitive species, represented today by P. missouriensis, 
moved early into the northward-flowing streams of the Missouri 
