Silurian
Period in immense numbers and under a greatly increased variety
of forms. The three prominent Lower Silurian genera _Orthis,
Strophomena_, and _Leptoena_ are still well represented, though
they have lost their former preeminence. Amongst the numerous
types which have now come upon the scene for the first time,
or which have now a special development, are _Spirifera_ and
_Pentamerus_. In the first of these (fig. 69. b, c), one of
the valves of the shell (the dorsal) is furnished in its interior
with a pair of great calcareous spires, which served for the
support of the long and fringed fleshy processes or "arms" which
were attached to the sides of the mouth.[16] In the genus
_Pentamerus_ (fig. 70) the shell is curiously subdivided in its
interior by calcareous plates. The _Pentameri_ commenced their
existence at the very close of the Lower Silurian (Llandovery),
and survived to the close of the Upper Silurian; but they are
specially characteristic of the May Hill and Wenlock groups,
both in Britain and in other regions. One species, _Pentamerus
galeatus_, is common to Sweden, Britain, and America. Amongst
the remaining Upper Silurian Brachiopods are the extraordinary
_Trimerellids_; the old and at the same time modern _Linguloe,
Discinoe_, and _Cranioe_; together with many species of _Atrypa_
(fig. 68, e), _Leptocoelia_ (fig. 68, a), _Rhynchonella_
(fig. 68, b, c), _Meristella_ (fig. 69, a, e, f), _Athyris,
Retzia, Chonetes_, &c.

[Footnote 16: In all the Lamp-shells the mouth is provided with
two long fleshy organs, which carry delicate filaments on their
sides, and which are usually coiled into a spiral. These organs
are known as the "arms," and it is from their presence that the
name of "_Brachiopoda_" is derived (Gr. _brachion_, arm; _podes_,
feet). In some cases the arms are merely coiled away within the
shell, without any support; but in other cases they are carried
upon a more or less elaborate shelly loop, often spoken of as the
"carriage-spring apparatus." In the _Spirifers_, and in other ancient
genera, this apparatus is coiled up into a complicated spiral (fig.
67). It is these "arms," with or without the supporting loops or
spires, which serve as one of the special characters distinguishing
the _Brachiopods_ from the true Bivalves (_Lamellibranchiata_).]

[Illustration: Fig. 69.-a a', Meristella intermedia_, Niagara
Group, America; b, _Spirifera Niagarensis_, Niagara Group, America;
c c', _Spir