ORDER III. Protremata (Beecher).--Articulate Brachiopoda, with
pedicle-opening restricted to ventral valve, and either open at the
hinge line or more or less completely closed by a pseudo-deltidium,
which may disappear in adult. The pro-deltidium originating on the
dorsal surface later becomes anchylosed with the ventral valve.
FAMILIES.--KUTORGINIDAE, EICHWALDIIDAE, BILLINGSELLIDAE,
STROPHOMENIDAE, _THECIDIIDAE_, PRODUCTIDAE, RICHTHOFENIDAE, ORTHIDAE,
CLITAMBONITIDAE, SYNTROPHIIDAE, PORAMBONITIDAE, PENTAMERIDAE.
ORDER IV. Telotremata (Beecher).--Articulate Brachiopoda, with the
pedicle-opening, confined in later life to the ventral valve, and
placed at the umbo or beneath it. Deltidium present, but no
pro-deltidium. Lophophore supported by calcareous loops, &c.
FAMILIES.--PROTORHYNCHIDAE, _RHYNCHONELLIDAE_, CENTRONELLIDAE,
_TEREBRATULIDAE_, STRINGOCEPHALIDAE, MEGALANTERIDAE,
_TEREBRATELLIDAE_, ATRYPIDAE, SPIRIFERIDAE, ATHYRIDAE.
_Affinities_.--Little light has been thrown on the affinities of the
Brachiopoda by recent research, though speculation has not been
wanting. Brachiopods have been at various times placed with the
Mollusca, the Chaetopoda, the Chaetognatha, the Phoronidea, the
Polyzoa, the Hemichordata, and the Urochordata. None of these
alliances has borne close scrutiny. The suggestion to place
Brachiopods with the Polyzoa, _Phoronis, Rhabdopleura_ and
_Cephalodiscus_, in the Phylum Podaxonia made in _Ency. Brit._ (vol.
xix, ninth edition, pp. 440-441) has not met with acceptance, and
until we have a fuller account of the embryology of some one form,
preferably an Inarticulate, it is wiser to regard the group as a very
isolated one. It may, however, be pointed out that Brachiopods seem to
belong to that class of animal which commences life as a larva with
three segments, and that tri-segmented larvae have been found now in
several of the larger groups.
_Distribution._--Brachiopods first appear in the Lower Cambrian, and
reached their highest development in the Silurian, from which upwards
of 2000 species are known, and were nearly as numerous in the Devonian
period; at present they are represented by some 140 recent species.
The following have been found in the British area, as defined by A.M.
Norman, _Terebratulina caput-serpentis_ L., _Terebratula (Gwynia)
capsula_ Jeff., _Magellania (Macandrevia) cranium_ Mull., _M.
septigera_ Loven, _Terebratella spitzbergenensis_ Dav., _Megathyris
decollata_ Chemn., _Cistella cistellula_ S. Wood, _Cryptopora gnomon_
Jeff., _Rhynchonella (Hemithyris) psittacea_ Gmel., _Crania anomala_
Mull., and _Discinisca atlantica_ King. About one-half the 120
existing species are found above the 100-fathoms line. Below 150
fathoms they are rare, but a few such as _Terebratulina wyvillei_ are
found down to 2000 fathoms. _Lingula_ is essentially a very shallow
water form. As a rule the genera of the northern hemisphere differ
from those of the southern. A large number of specimens of a species
are usually found together, since their only mode of spreading is
during the ciliated larval stage, which although it swims vigorously
can only cover a few millimetres an hour; still it may be carried some
little distance by currents.
Undue stress is often laid on the fact that _Lingula_ has come down to
us apparently unchanged since Cambrian times, whilst _Crania_, and
forms very closely resembling _Discina_ and _Rhynchonella_, are found
from the Ordovician strata onwards. The former statement is, however,
true of animals from other classes at least as highly organized as
Brachiopods, e.g. the Gasteropod _Capulus_, whilst most of the
invertebrate classes were represented in the Ordovician by forms which
do not differ from their existing representatives in any important
respect.
A full bibliography of Brachiopoda (recent and fossil) is to be found
in Davidson's Monograph of British Fossil Brachiopods, _Pal. Soc.
Mon._ vi., 1886. The Monograph on Recent Brachiopoda, by the same
author, _Tr. Linn. Soc. London_, Zool. ser. ii. vol. iv., 1886-1888,
must on no account be omitted. (A. E. S.)