Example shells from (a) Physa carolinae and (b) Physa acuta.

The six linear measurements are shown on shell a: Shell length (SL), shell width (SW), body whorl length (BWL), penultimate whorl width (PWW), aperture length (AL) and aperture width (AW). The 11 landmarks are located on shell b. Landmark points follow Dunithan et al. [47] and include points to capture the apex, aperture, lip, body whorl, and spire whorls. Landmarks on main body whorl and aperture lip were treated as sliding semilandmarks.