OF NORTH AMERICA. 245 



GENITALIA (pi. XI, fig. D) : Male organs: Penis small, narrow, 

 with rounded head, two-thirds as long as penis-sac, which is of large 

 diameter ; retractor muscles of penis one mill, in length, very slender ; 

 penis-sac retractor 1.25 mill, long and about twice as wide as the penis 

 retractor ; these muscles have their insertion close together in the colu- 

 mellar muscle ; protractor muscles four in number, two wide, powerful 

 anterior muscles entering the penis-sac by several branches, and two 

 narrow posterior protractors ; vas def erens four mill, in length ; pros- 

 tate, elongate, ovate, flattened, rounded at both ends ; the anterior end 

 gradually narrows to meet the prostate duct, which is about one mill, 

 in length. 



Female organs: Receptaculum seminis roundly pyriform, rather 

 large, its duct 1.75 mill, in length; first accessory albuminiparous gland 

 long-ovate, placed near the vaginal opening. 



The organs of parva are very uniform. In several specimens 

 examined, only one showed any variation and in this one the penis 

 retractor was attached to the penis-sac retractor a short distance from 

 its insertion in the columella muscle. (PI. XIV, fig. E, 2.) 



Three specimens gave the following measurements : 



Penis- Penis Penis- Rec. Prost. Vas. 

 Penis. sac. ret. sac. ret. isem. duct. def. Shell. 

 1.00 1.50 1.00 1.25 1.75 1.00 4.00 8.10 Des Moines (1) 

 1.00 1.50 1.00 1.25 1.75 1.00 3.00 7.00 (2) 



1.00 1.50 1.00 1.15 1.75 1.00 4.00 6.80 (3) 



The measurements show great uniformity. In one specimen (No. 

 2) the prostate duct was longer and the vas def erens shorter than in 

 the other specimens examined. (Dissections Nos. 23133 and 23132.) 

 The genitalia of parva are almost identical with those of umbilicata. 



RANGE (Figure 22) : Connecticut west to Idaho; James Bay and 

 Montana south to Maryland, Kentucky, Oklahoma, southern New 

 Mexico and Arizona. 



Parva is a characteristic species of the Upper Mississippian region, 

 from whence it has migrated into the Canadian, Hudsonian, Columbian 

 (via the Missouri-Columbia drainages), Coloradoan and Rio Grandian 

 regions. In the East it has penetrated into the Nova Scotian region. 

 Parva occupies a large part of the Canadian, Transition and Upper 

 Austral life zones. It is absent, apparently, from the Gulf and South 

 Atlantic states. Its metropolis appears to be the Upper Mississippi 

 Valley and the Great Lakes region. It has been universally confused 

 with humilis and modicella. When it is differentiated from these 

 species it will doubtless be found to be widely distributed. 



GEOLOGICAL RANGE: Pleistocene. 



