5 
Group of Helisoma Corpulentum 
Helisoma corpulentum (Say) 
Plate I, figures 1-7; Plate IV, figures 1-4 
Planorbis corpulentus Say, Long’s Exped., II, p. 262; PI. xv, Fig. 9 
(1824) ; Binney, Complete Writings of Thomas Say on Conch, of 
U.S., p. 128, PI. 74, Fig. 9 (1858) ; Binney, L. and F.-W. Shells N.A., 
II, p. 114, Fig. 190 (1865) ; Walker, Nautilus, XIII, p. 123, PI. iii, 
Figs. 1-3 (1900); Dali, Alaska Moll, p, 87, Fig. 65 (1905). 
Helisoma corpulenta (Say), Baker, Fresh Water Moll. Wis., I, p. 337, 
PI. xix, Figs. 40-42 (1928). 
Helisoma corpulentum Baker, Nautilus, XLVI, p. 6 (1932). 
Shell light to dark horn coloured, whorls about 4£, flatly rounded at 
the periphery, sharply carinated above and below; spire flat, usually 
sunken below the enveloping body whorl; umbilical region deep, less than 
three whorls visible; aperture reversed ear-shaped, higher than wide, 
sharply angled above, rounded below; lip reflexed and aperture entire, 
joined by a wide, heavy, white callus; the last half of the body whorl 
projects abruptly upward, throwing the aperture out of line with the rest 
of the shell ; sculpture of coarse, rib-like wrinkles or costa? normally spaced 
a millimetre apart, each rib heavy and raised above the level of the body 
whorl; there are fine horizontal lines parallel with and between the ribs, 
and fine transverse lines over the whole shell. 
H.ll-2; Gr. diam. 25-1; Ap.H. 11-0; D.7*5 mm., Rainy lake, Ont. Baker coll. 1101. 
H.12-5; Gr, diam. 21-5; Ap.H. 11*2; D.7*8 mm., Rainy lake, Ont. Baker coll. 1101. 
H.13-0; Gr. diam. 24*5; Ap.H. 12*5; D.8*5 mm., lac la Croix, Ont. U.I. coll. 
Z32096. 
H.14-0; Gr. diam. 25*1; Ap.H. 12*6; D.7*4 mm., lac la Croix, Ont. U.I. coll. 
Z32296. 
H.ll -5 ; Gr. diam. 19*0; Ap.H. 11*2; D.7-1 mm., lac la Croix, Ont. U.I. coll. 
Z32296. 
The last measurement is about the size of Say’s original specimen. 
In his original description Say mentions the shell as “ rather nigged 
with coarse wrinkles ” and it is this form that must be taken as represent- 
ing typical corpulentum. Specimens from Rainy lake (from the collection 
of Hon. Mr. Justice F. R. Latchford), which is one of original localities, 
are of this nature, with coarse, widely spaced rib-sculpture (Plate I, 
figure 6). Specimens from lac la Croix, southeast of Rainy lake, are 
exactly like Say’s diagnosis. One specimen (Plate I, figure 4), is like 
Say’s figure and about the size of his specimen. Although the majority 
of specimens of typical corpulentum will conform closely to the above 
diagnosis there are occasional specimens that have a greater axial height, 
in this respect approaching the race vermilionense (Plate I, figure 5). 
Also, individual specimens may have finer sculpture, approaching the race 
mvlticostatum. These variations are rare, however, and the majority of 
colonies are quite uniform and easily distinguishable as typical corpu- 
lentum. 
