Sectioning a seashell longitudinally can reveal some interesting internal details, and these can be highly useful for the identification purposes. In the case of Telescopium, notice the presence of an indentation (fold) on the inside of the aperture (the large opening at the anterior end of the shell). This fold is also present on most of the height of the central axis (columella) of the interior of the shell.
Exterior and interior views of modern-day specimens of Telescopium telescopium (Linnaeus, 1758) from Indo-Pacific mangrove mud flats. Height of shells approximately 3 inches.
Although Turritella superficially resembles Telescopium, the columella of Turritella does not have an indentation. The aperture of Turritella is round and without a fold.
Exterior and interior views of modern-day specimens of Turritella cerebra (Linnaeus, 1758) from Indo-Pacific shallow-marine waters. Height of shells approximately 4.5 inches.