the delthyrium. In some less primitive genera, e.g. _Terebratula_,
that type of opening is found in the young stages only; later it
becomes partly closed by two plates which grow out from the sides of
the delthyrium. These plates are secreted by the ventral lobe of the
mantle, and were named by von Buch in 1834 the "deltidium." The form
of the deltidium varies in different genera. The two plates may meet
in the middle line, and leave only a small oval opening near the
centre for the pedicle, as in _Rhynchonella_; or they may meet only
near the base of the delthyrium forming the lower boundary of the
circular pedicle-opening, as in _Terebratula_; or the right plate may
remain quite distinct from the left plate, as in _Terebratella_. The
pro-deltidium, a term introduced by Hall and Clarke, signifies a small
embryonic plate originating on the dorsal side of the body. It
subsequently becomes attached to the ventral valve, and develops into
the pseudo-deltidium, in the Neotremata and the Protremata. 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. The pseudo-deltidium is sometimes reabsorbed in
the adult. In the Telotremata neither pro-deltidium nor
pseudo-deltidium is known. In the Atremata the pro-deltidium does not
become fixed to the ventral valve, and does not develop into a
pseudo-deltidium. The American use of the term deltidium for the
structure which Europeans call the pseudo-deltidium makes for
confusion. The development of the brachial supports has been studied
by Friele, Fischer and Oehlert. A summary of the results is given by
Beecher (_Trans. Connect. Acad._ ix., 1893; reprinted in _Studies in
Evolution_, 1901).

The orders Atremata and Neotremata are frequently grouped together, as
the sub-class Inarticulata or Ecardines--the Tretenterata of
Davidson--and the orders Protremata and Telotremata, as the Articulata
or Testicardines--the Clistenterata of Davidson. The following scheme
of classification is based on Beecher's and Schubert's. Recent
families are printed in italic type.

[FIG. 32.--Diagram of the pedicle-opening of _Rhynchonella_. Magnified.

1. Umbo of ventral valve.
2. Deltidium.
3. Margin of delthyrium.
4. Pedicle-opening.
5. Dorsal valve.]

Class I. ECARDINES (INARTICULATA)

ORDER I. Atr