Gyraulus (Torquis) parvus is the snail intermediate host of trematode parasites infecting several species of owls and rodents in North America. Researchers often misidentify the species comprising Gyraulus, and its subgenus Torquis as well, due to a general lack of knowledge of dependable distinguishing morphological characters. Diagnostic characters in the anatomy of the subgenus Torquis are the sigmoidly folded kidney with undulating margins, the prostate diverticula arranged in a single row, and the straight, uncoiled intestine. The anatomy of the three North American species of Torquis is very similar, especially that of G. parvus and G. circumstriatus. The anatomical differences between these two species seem to be only general ones, relating to external pigmentation. Individual and populational variation might well render these pigmentation patterns valueless as diagnostic characters between the two species. Gyraulus huronensis is similar to the two preceding species, but there are distinct differences in external pigmentation, in the direct connection of the prostate gland diverticula to the sperm duct, in the overall shape of the prostate gland, and in the number of prostate gland diverticula (ca. 13-16 for G. huronensis vs. 7-10 for the other two species). The three species of Torquis exhibit several specific differences in radular teeth. Gyraulus parvus shows a general lack of inter-cusp denticles. This condition is even more noticeable in G. huronensis, which also has characteristically flatter, narrower lateral teeth with longer, broader mesocones. These radular differences may reflect evolutionary changes associated with different substrates and food. Gyraulus circumstriatus was characterized by the presence of denticles between the cusps on the central and lateral teeth. Also noticeable in G. circumstriatus were individual differences in radular teeth, two morphotypes occurring in the same population. These may be genetic differences associated with the peculiar populational breeding structure in some populations.