Diagnostic characters: The cross-furrow is nearly central (see,
however, _Oxytoxum_); the body may or may not have a shell; the
shell may or may not be composed of distinct plates; the plates are
distinguished as _equatorial_ (_i.e._, bordering the cross-furrow),
_apical_, and _antapical_, while still another, the "rhombic plate",
may be present, extending from the cross-furrow to the apex.