13 
region in Ottawa river, but that the form in Rideau river is injracarinatum 
{See PI. Ill, figures 7, 8, Ottawa trivolvis ), These small, flat specimens are 
possibly the form called lentus by Whiteaves. The trivolvis listed by 
Whiteaves (1863, page 104) are larger and heavier than this shell and are 
the form now called infracarinatum. The large collection of Helisoma 
trivolvis in the Royal Ontario Museum of Zoology shows the distribution 
of this species to cover all of lower Ontario from bay of Quinte west to lake 
Huron and Georgian bay (about twenty lots). It is also in the United 
States National Museum from lake Simcoe (No. 47555). The material 
in the Royal Ontario Museum is of special interest in showing great 
variation in the sculpture of the umbilical region, in several cases a lot 
from one locality exhibited all gradations from the typical rounded inner 
whorls to a condition in which the inner whorls had a sharp carina with 
several spiral striae, varying toward the forms of the Mississippi valley called 
pseudotrivolvis (F. C. Baker) and lentum (Say). A few of these lots are 
listed below: 
Georgian bay, Ontario, A. D. Robertson, collector. No. 1653. No. 1660. 
Turkey point, lake Erie, J. P. Oughton, collector. No. 6174. 
Hanlan point, Toronto, J. P. Oughton, collector. No. 1654. 
Hamilton, Ontario, J. P. Oughton, collector. No. 6478. 
Some of the Georgian Bay specimens have a diameter of 23 mm. 
Trivolvis appears to be rare in western Ontario, none having occurred 
in any of the Calm collections obtained in that region. It evidently lives 
abundantly farther north as typical examples occurred in collections from 
Wainwright park, Alberta (Animal Diseases Research Institute). This is 
the farthest north and west yet reported of the typical form. 
References to trivolvis from the interior of Canada and west of the 
Rocky mountains in the United States are based on other species. As these 
related species may (or have) been found in Canada it seems desirable to 
indicate the principal characters by which they may be known. 
Helisoma plexatum (Ingersoll) 
Plate III, figures 9-14 
Planorbis plexata Ingersoll, U.S. Geol. and Geog. Surv, Terr., 1874, 
p, 402, text figures; Planorbis trivolvis of western writers generally. 
Type Locality. St. Mary lake, Mineral county, Colorado. Types: 
lectotype, U.S. Nat. Mus., No. 420210. Paratypes: U.S.N.M., No. 125130. 
This Helisoma is usually higher axially than trivolvis, there are five 
spire whorls, the inner 2^ whorls more or less angled in the middle, the 
outer whorls rounded; the base shows rather more than three full turns, 
the inner whorls rounded, not angulated or spirally ridged; umbilicus small; 
sculpture of crowded riblets (3 to 4 in mm. on body whorl) ; the spire wdiorls 
are not sunken and flattened as in trivolvis, but are loosely coiled leaving 
a distinct suture between the rounded whorls, which are never marked by a 
sharp carina at the shoulder as in trivolvis; aperture much higher than wide, 
somewhat lunate, roundly angled above and below, extending well above 
and below the penultimate whorl, often widely expanded; outer lip thickened 
and bordered by a dark purple band in the mature shell. 
