At least three of the forms described by Brady (1884) as Amphistegina lessonii show specimens with monocular as well as bilocular nucleoconches, probably representing the microspheric and macrospheric generations. Except for the shape of the embryonic apparatus both generations are identical or practically so. Assuming that dimorphism occurs in Amphistegina, the above suggests that several species should be distinguished in Brady's Amphistegina lessonii. The internal structure of A. lessonii is illustrated by photographs and schematic drawings.