16 PROCEEDINGS OF THE NATIONAL MUSEUM VOL, 125 
resembles an ental process, except that often it is distended with 
spermatazoa. The wall of the spermathecal bulb is thicker than in all 
other species of the genus except P. simondsi. 
VARIATION.—One or two specimens have only three dorsal pro- 
jections on segment VIII instead of four. The length of the prongs 
vary, those of the type are larger than those of most specimens. This 
difference appears to be of sporadic occurrence and of no systematic 
importance. The prostate of some specimens is partially vacuolated 
(cf. P. simondsi, p. 25, below). 
AFFINITIES.—Pterodrilus mexicanus is related to P. missouriensis, 
P. choritonamus, P. cedrus, and P. hobbst. In features of the repro- 
ductive system, P. mezxicanus is most similar to P. missouriensis. 
These two Ozarkian species differ in that in P. mexicanus the ejacu- 
latory duct is short, the bursa is larger, the prostate is partially 
differentiated, the spermatheca is shorter and the wall of the sper- 
mathecal bulb is thicker, and there are no dorsal ridges on segments 
J-VII. The absence of dorsal ridges except on segment VIII ally 
P. mexicanus with P. hobbst, a more advanced member of the same 
lineage (see below, p. 36), which differs from P. mezxicanus in the 
fully differentiated prostate, long ejaculatory duct, small bursa, and 
thin-walled spermathecal bulb. Pterodrilus mezxicanus shares the 
absence of dorsal ridges, except on segment VIII, with P. choritonamus, 
which, however, lacks projections on this dorsal ridge. In addition, 
the latter species differs from P. mexicanus in that the bursa is smaller, 
the ejaculatory duct is longer, the prostate is differentiated, and the 
spermatheca has an ental process. Pterodrilus cedrus belongs in the 
same lineage as P. missourvensis and differs from P. mezxicanus in the 
presence of dorsal ridges on segments I-VIJ, a smaller, more nearly 
spherical bursa, a longer ejaculatory duct, a differentiated prostate, and 
a longer spermatheca without an ental process. 
Hosts.—Pterodrilus mezicanus has been taken from 10 species of 
the genus Orconectes: O. punctimanus (Creaser), O. ozarkae Williams, 
O. mecki meeki (Faxon), O. neglectus neglectus (Faxon), O. nana nana 
Williams, O. nais (Faxon), O. luteus (Creaser), and O. hylas (Faxon). 
DistrisuTion.—Two of my collections are from the Arkansas 
River drainage in northwestern Arkansas and eastern Oklahoma; 
four are from the St. Francis River system in Missouri; the remainder 
are from the White River drainage in Arkansas and Missouri. All of 
these streams, however, drain the Ozark highlands centered in south- 
central Missouri. The range of P. mezicanus, thus, is compact and 
well delimited except for the type-locality, Veracruz, Mexico. 
MATERIAL EXAMINED.—Approximately 100 specimens from 22 
collections from Arkansas, Missouri, and Oklahoma were studied. 
