14 
H.lll'O; Gr. diam. 22-0; Ap.H. 10-0; D.7*0 mm. Lectotype, St. Mary lake. 
H.13*0; Gr. diam. 28-5; Ap.H. 12- 0; D.8-0 mm. Grand Mesa, Colo. 
H.10’0; Gr. diam. 22*0; Ap.H. 8*5; D.6-5 mm. Grand Mesa, Colo. 
Gr. diam. 22-7; Ap.H. 10*2; D.6*4 mm. Grand Mesa, Colo, 
The shell diagnosed above is common in Colorado and has been 
observed in Idaho, Montana, and Washington. It is probably widely dis- 
tributed in the western states. It differs uniformly from trivolvis, both in 
shell and radula, It extends northward into Canada. Whiteaves’ record 
of trivolvis (Nautilus, XIX, page 4) from 2 miles above the mouth of 
Harricanaw river, Hannah bay, collected by W. Spreadborough in July 
1904, is plexatum (Plate III, figures 11, 12). The following records in the 
National Museum of Canada are also referable to plexatum : Black Bear 
island, lake Winnipeg, collected by Dowling and Lambe (Plate III, figures 
13, 14) ; Cormorant lake, Manitoba, collected by W. Mclnnes in 1906. 
Specimens have been seen from Athlestane lake, Ontario, collected by 
Dr. A. R. Cahn, of the University of Illinois. Material in the Haines col- 
lection of the American Museum of Natural History (A3868), however, is 
typical trivolvis, said to have come from the “ Winnipeck ” river. Material 
from this region is needed to satisfactorily interpret the distribution of 
trivolvis and plexatum. The extent of the distribution of plexatum in 
Canada is at present unknown, but probably covers a wide area. In many 
specimens the whorls are distorted, as noted by Ingersoll in his original 
description. This, however, is not a specific character, for it has been 
observed in a number of species. 
Helisoma hornii (Tryon) 
Planorbis hornii Tryon, Am. Jour. Conch., I, p. 231, PI. 22, fig. 16 
(1865). 
This large planorbid snail has been somewhat of a puzzle to American 
conchologists since its publication. There has been doubt about the type 
locality as to whether the Simpson mentioned by Tryon was on Mackenzie 
river in Canada or on Portland sound in the western part of British 
Columbia (Dali, Alaska Moll., p. 89). Cooper (Proc. Cal. Acad., 2, III, 
p. 88, 1890) says, however, “ hornii was described from the coast at the 
southern boundary of Alaska, latitude 54° 40' ” and this seems to have been 
the location accepted by the Californian conchologists of this period. Dr. 
Pilsbry also believes that this was the locality, as the Philadelphia Academy 
has many specimens received from Dr. Horn from this region but none 
from Mackenzie River region. As Dr. Horn was a United States army 
surgeon it is extremely unlikely that he should have been stationed on 
Mackenzie river in Canadian territory. There appears to be no reasonable 
doubt about Tryon's specimens coming from Port Simpson, on Portland 
sound, British Columbia, and this type locality should be used by all 
workers on Canadian Mollusca. 
Just what the species is that Tryon called hornii has also been in 
considerable doubt. Dali and Cooper relate it to subcrenatum (Cpr.) , but 
the mature shell of the latter species is quite unlike Tryon's figure and 
description. Henderson and Daniels (Proc. Phil. Acad., 1917, p. 65) list 
