ORDER I. Atremata (Beecher).--Inarticulate Brachiopoda, with the
pedicle passing out between the umbones, the opening being shared by
both valves. Pro-deltidium attached to dorsal valves.
FAMILIES.--PATERINIDAE, OBOLIDAE, TRIMERELLIDAE, LINGULELLIDAE,
_LINGULIDAE_, LIGULASMATIDAE.
ORDER II. Neotremata (Beecher).--More or less circular, cone-shaped,
inarticulate Brachiopoda. The pedicle passes out at right angles to
the plane of junction of the valves of the shell; the opening is
confined to the ventral valve, and may take the form of a slit, or may
be closed by the development of a special plate called the listrium,
or by a pseudo-deltidium. Pro-deltidium attached to ventral valve.
FAMILIES.--ACROTRETIDAE, SIPHONOTRETIDAE, TREMATIDAE, _DISCINIDAE_,
_CRANIIDAE_.