404 GEOLOGICAL SURVEY OF TIIE TERRITORIES. 



and the causes which lead to their production, than by the study of such 

 isolated localities, where causes are reduced to their least complicated 

 state, and results reach their maximum, so far as the observability and 

 variety of the characteristics are concerned." 



Helisoina trivolvis, Say. 



Pond between the Animas and La Plata 10 specimens. 



San Luis Lakes 5 specimens. 



There is a long list of recorded localities from all the Territories, as 

 well as British America and tbe Pacific coast, so that it seems univer- 

 sally distributed over this continent. 



The pond first alluded to was entirely isolated, and several acres in 

 extent, resorted to by vast flocks of wild fowl, and inhabited by all sorts 

 of fresh-water and amphibious life. The bottom was muddy, and nearly 

 the whole expanse choked with luxurious vegetation. 



All of the species, which were abundant, seem to belong to this 

 species, although there are scarcely two alike. One resembles closely 

 Planorbis macrostomus, Whiteaves; another is near P. tumens, Cpr.; a 

 third variety might be identified as P. glabratus, Say, if that shell were 

 dextral ; yet, while all differ in development and in color, all agree in 

 being very fragile, which may be owing partly to scarcity of lime in the 

 water, and partly to the soft bottom; and in having a short vertical 

 diameter, which peculiarity may have been acquired by them from the 

 necessities of their habitat, since snails having shells with small breadth 

 of beam could most advantageously pass between the stalks of standing 

 water-plants which everywhere crowd the pond. This species is an 

 inhabitant of the United States generally. 



Helisoma ammon, Gld. — Colorado Desert {Blake) ; Klamath Lake, Ore- 

 gon, and Bhett Lake, California {Newberry) ; east of Fort Colville, Wash- 

 ington Territory {Northwestern Boundary Survey). Pacific slope. 



Helisoma Traskei, Lea, is probably a synonym of the above. Cali- 

 fornia. 



Helisoma corpulentus, Say. — Oregon and Washington Territory ( United 

 States Exploring Expedition) ; Lake Osoyoos, Washington Territory 

 {Marsh) ; British Columbia {Lord) ; Okanigan Biver, Washington Terri- 

 tory {Cooper) ; Pacific Coast and Guatemala. It seems doubtful whether 

 this is not a synonym of H. trivolvis. 



Planorbis oregonensis, Tryon.— Pueblo Valley, on the boundary be- 

 tween Oregon and Nevada, " from a thermal spring, water above blood- 

 heat" {Gabb). 



Planorbis subcrenatus, Cpr.— Oregon {Nuttall) ; Washoe, Nevada {New- 

 comb) ; Sumass Prairie, British Columbia {Lord). California. 



Planorbis Hornii, Tryon. — Utah {Su.rv. W. of 100th If.); Truckee Kiver, 

 Nevada {Carlton). 



Planorbis gracilentus, Gld. — Colorado Desert ( Webb). This seems to 

 be a northern form of P. Liebmanni, Dunker. 



Gyraulus parvus, Say. 



Camp 9: Hot Sulphur Springs 1 specimen. 



Arkansas River, ten miles below Granite - . 5 specimens. 



Between Bio Animas and Bio La Plata 2 specimens. 



Saint Mary's Lake, Antelope Park 50 specimens. 



North Park {Barber) , 6 specimens. 



