The form of the deltidium varies in different genera.
The pro-deltidium, a term introduced by Hall and Clarke, signifies a small embryonic plate originating on the dorsal side of the body.
The pseudo-deltidium (so named by Bronn in 1862) is a single plate which grows from the apex of the delthyrium downwards, and may completely close the aperture.
In the Telotremata neither pro-deltidium nor pseudo-deltidium is known.