NO. 3668 PTERODRILUS—HOL' 15 
where near the Kentucky River across the Ohio into the glaciated 
areas of the Ohio-Mississippi and Great Lakes drainage systems since 
the melting of the Wisconsin glacier. 
MarterraL Examrnep.—Approximately 200 specimens from 25 
collections have been examined. Specimens from all these collections 
are deposited in the U.S. National Museum (USNM 17651-17653, 
36160-36183). 
Pterodrilus mexicanus Ellis 
FIGURES 3, 9 
Pierodrilus mexicanus Ellis, 1919, p. 254.—Goodnight, 1940, p. 63.—Causey, 
1955, p. 44. 
TyYPE-sPECIMEN.—Holotype, USNM 17654, from Mirador, Vera- 
cruz, Mexico. Host: Cambarus mexicanus Erichson; Nelson and Gold- 
man, collectors. 
Draenosis.—Dorsal ridge on segment VIII, typically bearing four 
conical projections, remainder of segments without ridges; bursa large, 
elongate, length exceeding 4 body diameter; ejaculatory duct short; 
prostate about }; diameter of and subequal in length to spermiducal 
gland, undifferentiated; spermatheca shorter than body diameter, 
bulb thick walled. 
Description.—Pterodrilus mexicanus differs externally from other 
species of Pterodrilus in the arrangement and number of the dorsal 
ridges and projections. There are no ridges present, except that on 
segment VIII, which has four finger-like, conical projections, very 
similar to those of P. distichus. The total length averages 1.1 mm. 
The spermiducal gland is about three times its diameter in length 
and lies along the upper border of the gut. The prostate, subequal in 
length to and about half the diameter of the spermiducal gland, is 
histologically undifferentiated in most specimens although some 
specimens show a vacuolation of some cells along its ental and dorsal 
borders. The ejaculatory duct is very short. The bursa, however, is 
large, about 1% times longer than that of P. alcicornus and 3 times 
that of P. distichus. This great increase in size is primarily accounted 
for by an increase in the length of the atrial area, which is not only 
larger but has additional inwardly directed folds of the bursal wall. 
The penial sheath region and the penis itself is as in other species of 
Pterodrilus. Specimens with the bursa everted have not been seen; 
but one would expect a cup-within-a-cup structure to be produced by 
eversion. 
The spermatheca of P. mexicanus is shorter than that of most 
species of the genus, hardly extending above the upper border of the 
gut. The inner part of the ectal duct is often enveloped in an expanded 
ectal part of the bulb. The blind end of the spermatheca frequently 
