16 
three authentic localities at present known show its distribution to be from 
Port Simpson, British Columbia, northeast to Simpson on Mackenzie river, 
and southward to south-central British Columbia. 
Helisoma subcrenatum (Carpenter) 
Plate III, figures 15-16 
Planorbis subcrenatus Carpenter, Proc. Zool. Soe., 1856, p. 220; Binney, 
L. and F.-W. Shells N.A., II, p. 103, Fig. 176 (1865). 
This large, western species is practically unknown to present-day 
conchologists. Carpenter described the species from a single specimen col- 
lected by Nuttall in “ Oregon.' 7 As the Oregon of the early thirties included 
all of the territory west of the Rocky mountains and north of California it 
is impossible to know where Nuttall collected his specimen. The figure in 
Binney and the original description, however, appear to give a definite clue 
to the identity of the species when compared with a large series of western 
Helisoma. Its size, 0*95 inch in diameter, 0*36 inch in height (a typo- 
graphical error in Binney makes the shell 0*05 inch in diameter) indicates 
a large shell nearly an inch in diameter, and of low axial height. The 
manuscript note by Cummings, in Binney, also reveals a diagnostic feature, 
“ differs from Planorbis trivolvis Say, in the acuteness of the ribs, and in 
their being more distant. 77 In trivolvis there are 3 to 5 riblets in 1 mm., 
whereas in the western species believed to be subcrenatum there are 1 to 3 
riblets in the same space. This species, as understood by the writer, may 
be diagnosed as follows: 
Shell depressed, whorls 5 to 5^, spire whorls rounded or only slightly 
angulated, the nuclear whorls rounded, and all whorls separated by a deep 
suture; base showing three full turns of regularly and slowly increasing 
whorls, well rounded, a fourth whorl disappearing in the very deep 
umbilicus; sculpture of widely spaced, rather acute riblets, 1 to 3 in the 
space of 1 mm., often more than 1 mm. apart; aperture rounded, flattened 
below and rounded or bluntly angled above, usually a trifle wider than 
high, and frequently expanded; the aperture may be entire in some speci- 
mens, the parietal wall being covered by a white callus; interior of aper- 
ture brownish, often with a purplish border within the lip; colour of shell 
greenish or light horn. 
H. 9-0; Diam. 24*5 mm. 
H.10*0; Gr. diam. 21*0; Ap.H. 9*0; D.6*3 mm. 
H. 9*2; Gr. diam. 19*4; Ap.H. 8*6; D,6*l mm. 
H. 9-0; Gr. diam. 2T5; Ap.H. 8*0; D.7*0 mm. 
H.10-5; Gr. diam. 23*0; Ap.H. 9*0; D.7-5 mm. 
Carpenter’s measurements. 
Newton Town, Utah. 
Newton Town, Utah. 
Logan, Utah. 
Wainwright park, Alta. 
Helisoma subcrenatum differs from H. plexatum, with which it has 
been confused, in its generally flatter shell, more orbicular aperture, 
rounder spire whorls, and particularly by its more widely spaced sculpture, 
plexatum having 3 to 6 riblets in the space of 1 mm. Subcrenatum is 
also a much thicker shell, although Carpenter described it as “ very thin.” 
Some specimens are thin, but the majority are rather thick as compared 
