F, foramen; d, deltidium; t, teeth; a, adductor impressions (= occlusors, Hancock); c, divaricator (=cardinal muscles, King, = muscles diducteurs principaux, Gratiolet); c', accessory divaricators (muscles diducteurs accessoires, Gratiolet); b, ventral adjustor (=ventral peduncular muscles, or muscles du pedoncule paire superieure, Gratiolet); b', peduncular muscle.
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.
In the Atremata the pro-deltidium does not become fixed to the FIG.
The American use of the term deltidium for the structure which Europeans call the pseudo-deltidium makes for confusion.
Pro-deltidium attached to dorsal valves.
Pro-deltidium attached A B C FIG.
The pro-deltidium originating on the dorsal surface later becomes anchylosed with the ventral valve.
Deltidium present, but no pro-deltidium.