21 
Helisoma infracarinatum F. C. Baker 
Plate II, figures 6-15; Plate IY, figures 15, 16, 20 
Helisoma infracarinatum F. C. Baker, Nautilus, XLYI, p. 8 (1932). 
Planorbis binneyi Baker, Nautilus, XXIII, p. 41 (1909) (part). 
Planorbis trivolvis Whiteaves, Can. Nat. and Geol., VIII, p. 104 
(1863) ; and of most Canadian authors. 
Shell resembling Helisoma pilsbryi in general form, but distinguished 
by a more or less heavy carina on the centre of the basal whorls; the 
umbilical region is more sunken and usually exhibits three full whorls, 
whereas in pilsbryi the whorls shown are a trifle less than three; the spire 
whorls are carinated, the body whorl sharply carinated, whereas they are 
rounded or bluntly angled in pilsbryi; the aperture is greatly expanded 
below and forms a strongly angled V-shape above with the dorsal carina 
extending to the aperture, and the lip is heavily reflexed, features absent in 
pilsbryi ; infracarinatum also has a thicker shell; the sculpture is rib- 
striate, coarser than in pilsbryi; radula formula 32-1-32 to 37-1-37, whereas 
in pilsbryi it is 27-1-27 to 29-1-29. 
H. 14*0; Gr. diam. 25*5; Ap. H. 12-6; D. 8*5 mm. Holotype, U.I. coll. Z32361. 
H. 12*3; Gr. diam. 24-0; Ap. H. 11*0; D. 7*3 mm. Paratype, U.I. coll. Z32362. 
H. 12*3; Gr. diam. 23*0; Ap. H. 11*1; D. 7*2 mm. Paratype, UJ. coll. Z32362. 
H. 10*0; Gr. diam. 18*0; Ap. H. 9*5; D. 6*0 mm. Paratype, U.I. coll. Z32362. 
Type Locality. Basswood River rapids, Rainy River dist., Ontario, 
Canada. 
Types. Mus. Nat. Hist., U.I., holotype and paratypes, Z32361, 
Z32362; paratypes, Nat. Mus., Canada, No. 3222; Acad. Nat. Sci. Phil., 
158594. 
This species appears to be one of the most abundant Helisomas in 
certain parts of Canada. It has been confused with both corpulentum and 
pilsbryi, from both of which it is quite distinct. The differentiating 
characteristics separating it from pilsbryi have been noted above. From 
corpulentum it differs in having the carina in the centre instead of at the 
edge of the whorl and the sculpture is much finer ; from the race multicosta - 
turn it differs in having a rounded instead of a flattened base, a rounded 
instead of a flattened body wdiorl, and generally more rounded whorls above 
and below. Infracarinatum is liable to be confused with large forms of 
trivolvis which have a decided carina on the whorls of the upper surface. 
In the absence of the radula these shells may be distinguished by being of 
much shorter axial height, and by having an aperture wider than high, 
whereas it is the reverse in the present species. Such perplexing shells of 
trivolvis occur in the lakes of central Wisconsin. These shells differ from 
macrostomum in the possession of a sharp keel on the lower whorls, those 
of macrostomum being rounded. 
There is the most perplexing variation in the series of shells here re- 
ferred to infracarinatum. Specimens from Basswood River rapids and 
from Georgian bay are typical, with a well-developed carina in the centre 
