No. 3668 PTERODRILUS—HOLT 5 
thick and the prostate is less completely divided from the spermiducal 
gland than in any species of Cambarincola. 
The dorsal projections of such species as P. alcicornus, P. distichus, 
P. mexicanus and three of the five new species described herein readily 
set them apart from Cambarincola. There would be no difficulty in 
maintaining the generic separateness of these species except for the 
last two of the new species treated herein, which are closely related 
to the others but lack fan- or finger-like projections on the ridge of 
segment VIII. This is not unexpected: the species assigned to Ptero- 
drilus are believed to have arisen from a generalized stock of Cambar- 
incola as animals adapted to a niche that favored a reduction in size 
and the production of the ridges and their projections. 
The species of Péerodrilus are a distinct group that might be 
placed within a larger group which includes the species assigned at 
this time to Cambarincola. Since, however, generic status has been 
accorded these two groups for many years, I prefer to retain both 
names and assign such taxa as the new species without dorsal projec- 
tions to one or the other of the existing genera on the basis of judg- 
ments as to the closeness of affinities with species previously assigned 
to them. There are precedents for such decisions in many groups; 
for instance, among the hosts of the branchiobdellids, the genera 
Procambarus and Orconectes are united by intermediate species that 
must be assigned rather arbitrarily to either genus (Hobbs, 1967, 
pes): 
One species, P. durbini Ellis (1919, pp. 254-255), previously 
assigned to Pterodrilus has been removed from the genus and referred 
to the genus Ellisodrilus Holt (1960b, pp. 173-176). Ellisodrilus is one 
of a group of genera related to Cambarincola and hence to Pterodrilus. 
It differs from Pterodrilus in the absence of a spermatheca and the 
asymmetry and other unique features of the bursa. Ceratodrilus, 
Oedipodrilus, and Magmatodrilus are other related genera. Mag- 
matodrilus Holt (1967b) lacks a prostate, the bursa is proportionally 
quite large and there are no dorsal projections; the penial sheath 
of Oedipodrilus is elongated, enclosing an eversible penis, the prostate 
is relatively very small and dorsal projections are absent (Holt, 
1967a, p. 58); Ceratodrilus Hall (1914, p. 191) is composed of larger 
worms in which the prostate is extremely reduced in size and the 
penis is eversible (Holt, 1960a, p. 57). 
DisTRIBUTION.—The genus Pterodrilus is confined to eastern 
North America including Mexico. Within this area there are three 
distinct centers of distribution: the Southern Appalachians with 
adjacent portions of the Interior Plateau east of the Mississippi 
River and the glaciated region north of the Ohio River to the 
Great Lakes and the Saint Lawrence River; the Ozarkian uplift 
