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,