rm, however,
is the curious _Archimedes_ (fig. 126, c), which is so
characteristic of the Carboniferous formation of North America.
In this remarkable type, the colony consists of a succession of
funnel-shaped fronds, essentially similar to _Fenestella_ in
their structure, springing in a continuous spiral from a strong
screw-like vertical axis. The outside of the fronds is simply
striated; but the branches exhibit on the interior the mouths of
the little cells in which the semi-independent beings composing
the colony originally lived.

[Illustration: Fig. 127.--Carboniferous _Braciopoda. a, _Producta
semireticulata_, showing the slightly concave dorsal valve; a'
Side view of the same, showing the convex ventral valve; b,
_Producta longispina_; c, _Orthis resupinata_; d, _Terebratula
hastata_; e, _Athyris subtilita_; f, _Chonetes Hardrensis_; g,
_Rhynchonella pleurodon_; h, _Spirifera trigonalis_. Most of
these forms are widely distributed in the Carboniferous Limestone
of Britain, Europe, America, &c. All the figures are of the natural
size. (After Davidson, De Koninck, and Meek.)]

The _Brachiopods_ are extremely abundant, and for the most part
belong to types which are exclusively or principally Palaeozoic
in their range. The old genera _Strophomena, Orthis_ (fig. 127,
c), _Athyris_ (fig. 127, e), _Rhynchonella_ (fig. 127, g),
and _Spirifera_ (fig. 127, h), are still well represented--the
latter, in particular, existing under numerous specific forms,
conspicuous by their abundance and sometimes by their size. Along
with these ancient groups, we have representatives--for the first
time in any plenty--of the great genus _Terebratula_ (fig. 127,
d), which underwent a great expansion during later periods,
and still exists at the present day. The most characteristic
Carboniferous Brachiopods, however, belong to the family of the
_Productidoe_, of which the principal genus is _Producta_ itself.
This family commenced its existence in the Upper Silurian with
the genus _Chonetes_, distinguished by its spinose hinge-margin.
This genus lived through the Devonian, and flourished in the
Carboniferous (fig. 127, f). The genus _Producta_ itself,
represented in the Devonian by the nearly allied _Productella_,
appeared first in the Carboniferous, at any rate, in force, and
survived into the Permian; but no member of this extensive family
has yet been shown to have over-lived the Palaeozoic period. The
_Productoe_ of the Carbonife