lymnmidje of north America. 281 



Nebraska : Five or six miles southwest of Santee Agency, Knox Co., in 

 stratum of laminated clay (Todd). 



New Mexico: Dry salt lake, near Eddy, Eddy Co. (Sterki). 



British America. 



Ontario: Interglacial beds, Don Valley, near Toronto, York Co. (Cole- 

 man). 



pliocene. 



California: Tassajora Lake bed, along small branch of Walnut Creek, 

 Alameda Co. (Cooper). 



Ecology: Found generally in small bodies of water, as creeks, 

 ponds, sloughs, bays and marshy spots along river banks. Frequently 

 inhabits abandoned quarries which have become filled with water. Like 

 Galba humilis modicella and Galba parva, this species is at home on 

 sticks, stones and exposed muddy surfaces (Baker) ; in warm water 

 streams from Hot Springs, Hill City, Black Hills (Ulke) ; on weeds, 

 Owasco Lake, New York (Lyon) ; "obrussa is abundant at Boulder, 

 in overflow ditches, in mud, the channels free from vegetation and, 

 I believe, in no case have I found it where water stands or flows the 

 year round" (Henderson) ; dredged in Lake Superior at a depth of 

 eight to fifteen fathoms (U. S. Fish Com). 



Remarks: Obrussa is one of the most widely distributed, as it 

 is one of the most variable, of the American Lymnaeas, not even ex- 

 cepting the circumboreal palustris. The spire may be long or short, 

 pointed or wide, and strongly shouldered. The whorls may be slender, 

 scalar or wide and corpulent, while the aperture varies from almost 

 round to long and narrow. The umbilicus may be rather widely open 

 or it may be reduced to a minute chink, depending upon whether the 

 inner lip is closely appressed to the axis or is raised over the umbilical 

 region. Imperforate specimens are, however, very rare. The fine 

 spiral sculpture can only be seen when the shell is examined with the 

 aid of a powerful lens ; in some specimens these lines are very distinct, 

 while in others they are scarcely discernable. Typically, obrussa may 

 be known by its pointed spire, compressed body whorl and elongated 

 and shouldered aperture, which is also strongly effuse at the anterior 

 end; the inner lip is appressed to the body whorl about the middle of 

 the aperture. The shape of the shell, of the aperture and of the inner 

 lip is quite different from Galba humilis modicella, the shell being 

 larger and more elongated, the last whorl not so convex ; the aperture 

 is longer and narrower and much more effuse, besides forming a dis- 

 tinct shoulder at its junction with the body whorl; the inner lip is 

 more compressed in the middle where it joins the parietal wall. In 



