15 
the species as Helisoma trivolvis hornii, identified by Dr. Pilsbry, from 
Newton Town reservoir, near Clarkston, Utah. Specimens of this lot 
recently examined are related to subcrenatum, having the sculpture and 
aperture of that species. Tryon emphasized the rotundity of the aperture 
and the roundness of the whorls “ equally convex above and below ”. 
Tryon’s types have apparently been lost. No specimens of the large 
Helisoma group are available from the region of Port Simpson, British 
Columbia, and material from the southern part of Vancouver island is 
quite unlike Tryon’s figure of hornii , having the body whorl disproportion- 
ately enlarged and the aperture expanded and slightly oblique. The 
sculpture is also coarser. This form is related to plexntum . 
Recently, several specimens of a large “ Planorbis ” were received 
from the U.S. National Museum, collected by Robert Kennieott at Simpson 
(No. 28375). These are listed as trivolvis by Binney on page 121 of the 
“ Fresh Water Shells,” Nos. 8952 and 9069. A study of Kennieott ’s journals 
proves beyond doubt that this locality is on Mackenzie river, where he 
remained one winter. There is no record of a visit to Simpson, British 
Columbia. These specimens fulfil the original diagnosis exactly and 
correspond with the figure. They have the rounded aperture and the 
whorls equally rounded above and below*. Measurements of three speci- 
mens are similar to the dimensions given by Tryon. 
H.8-9; Gr. diam. 22-5; Ap.H. 7*4; D.7-0 mm. Simpson (Kennieott). 
H.9-0; Gr. diam. 19-5; Ap.H. 8-0; D.5-6 mm. Simpson (Kennieott). 
H.8-0; Gr. diam. 19*5; Ap.H, 7*1; D.5-6 mm. Simpson (Kennieott). 
H.7*0; Diam. 21*0 mm. Tryon’s measurements, page 231. 
H.9-3; Diam. 21*0 mm. Dimensions of Tryon’s figure. 
It will be noted that the dimension given does not correspond in 
height with that of the shell figured. The dimensions of the figure cor- 
respond with those of the Kennieott specimens. Tryon’s figures in the 
“ Continuation of Haldeman’s Monograph ” arc different from those given 
in the American Journal of Conchology, being much smaller, but his 
description is the same in both. 
Quite recently, some specimens of a large Helisoma were received 
from Professor D. S. Raw^son, of the University of Saskatchewan, col- 
lected in Paul lake, in Kamloops region, south-central British Columbia. 
These are like Tryon’s description and figure and are nearer the type 
locality than the Mackenzie River specimens. Tw*o speciments measure 
as follows: 
H.8-5; Gr. diam. 18*0; Ap.H. 8*0; D.5*l mm. 
H.8-0; Gr. diam. 16*0; Ap.H. 6*6; D.4-5 mm. 
Tryon’s hornii appears to be a distinct species characterized by fine 
sculpture, the riblets, though widely spaced (2 to 3 in 1 mm., some 1 mm. 
apart) are low and inconspicuous, upper and low r er wdiorls equally rounded, 
the inner whorls of the spire with a distinct suture, the aperture rounded, 
regularly orbicular, without sharp angles above or below*. In these features 
it differs from any known species of Helisoma. The shell is also thin. The 
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