[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.
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