40 PROCEEDINGS OF THE NATIONAL MUSEUM VOL, 125 
Ozarks. Whether P. mexicanus is a descendant of this stock that moved 
over the Ozarkian divide into the White River and adjacent drainages 
cannot be determined with certainty: the postulated phylogeny sug- 
gests that it hkewise came into the Ozarks by the same route, but if 
so, it seems somewhat strange that it is unknown from the Missouri 
basin. Yet few collections have been taken from the northern Ozarks 
in Missouri, and further field work may well reveal the presence of 
P. mexicanus there. But P. mezicanus is obviously extinct over much 
of the route it or its ancestral form must have taken to reach the 
Ozarks, and the same or similar factors that caused this restriction of 
range may operate in the streams of the Missouri River system in 
southern Missouri. It is not surprising that an early stock of Ptero- 
drilus may have moved into Mexico: the crayfish hosts did so some- 
time before the end of the Miocene (Hobbs, 1967, p. 15). The possi- 
bility remains that the Ozarkian worms are not conspecific with the 
type of P. mexicanus (see p. 18 above), but any solution of the problem 
of the status of P. mezicanus will fit these ideas; for if it is recovered 
from Mexico and a new name assigned to my specimens from the 
Ozarks, the Mexican worms are, on the basis of my study of the type, 
very similar to the Ozarkian ones. Such a solution, however, would 
date the early migrations of Pterodrilus stocks in the Miocene or 
earlier (Hobbs, 1967, p. 15). 
Turning now to the north and east, we note that there are large 
gaps in the range of P. distichus (fig. 10) that can only be attributed to 
inadequate collecting. The records from the Kentucky and Licking 
Rivers are near the postulated place of origin of the species and may 
represent the Pleistocene refugium from which P. distichus has moved 
north and northeastward, most likely by way of the Miami and Scioto 
Rivers, since the Wisconsin glaciation. The gap in the range of P. 
distichus in the Lake Erie basin in Pennsylvania and New York surely 
represents inadequate collecting. 
Pterodrilus hobbsi has arisen from a stock that also produced P. 
mexicanus, but it has reached a higher level of development in the 
structures of the reproductive systems. Its sympatry with its prim- 
itive relative P. choritonamus argues for its origin in a part of the 
Cumberland basin, perhaps the headwaters of the Cumberland in 
southeast Kentucky, not inhabited by the latter and a reinvasion of 
the homeland. From such a region, the invasion of the Tennessee 
basin, where P. hobbsi is widespread and successful, of the Big Sandy, 
and of the New River is entirely possible. There are, however, gaps 
in its known range, and other histories of the species are possible. 
Its absence from the central part of the Cumberland Plateau in 
Tennessee appears to be real, but further collecting can be expected 
