NO. 3668 PTERODRILUS—HOLT 31 
Arrinitins.—Pterodrilus missourvensis is a primitive pterodrilid 
related to P. choritonamus, P. mexicanus, and P. cedrus (p. 21). It 
shares with P. cedrus the dorsal ridges of segments J-VIIT but differs 
in the absence of projections on the dorsal ridge of segment VIII, the 
undifferentiated prostate, the shape of the spermiducal gland, the 
larger size of its bursa, and in the thicker-walled spermathecal bulb. 
Pterodrilus missouriensis and P. choritonamus both lack dorsal projec- 
tions on segment VIII and have large bursae but differ in the presence 
of dorsal ridges on other segments, the undifferentiated prostate, the 
thicker-walled spermathecal bulb in the former, and an ental process 
mg 
| GA 
baa 
Yj. CHORITONAMUS 
a P. MISSOURIENSIS 
: P. MEXICANUS 
 P. HOBBSI 
Ficure 9.—Distribution of certain species of Pterodrilus. 
of the spermatheca in the latter. Pterodrilus missouriensis shares with 
P. mexicanus the primitive nature of the prostate (p. 15) (though that 
of the latter is often partially differentiated), the large size of the 
bursa, and a spermatheca with a thicker-walled bulb; it differs from 
the latter in its much longer ejaculatory duct, the presence of dorsal 
ridges on segments anterior to segment VIII, and in the absence of 
dorsal projections. 
Hosts.—The only known host is Orconectes luteus (Creaser). 
DistRiBuTion.—Pterodrilus missouriensis is known only from the 
type locality, Whetstone Creek in Wright County, Mo. The one 
collection was taken from shallow pools in the headwaters of the 
stream, where there was little or no flow at an elevation of approxi- 
mately 1260 feet. This locality is near the divide between the south- 
