(After M'Coy and
Hall.)]
The lower groups of the _Mollusca_ are abundantly represented
in the marine strata of the Carboniferous series by _Polyzoans_
and _Brachiopods_. Amongst the former, although a variety of other
types are known, the majority still belong to the old group of
the "Lace-corals" (_Fenestellidoe_), some of the characteristic
forms of which are here figured (fig. 126). The graceful netted
fronds of _Fenestella, Retepora_, and _Polypora_ (fig. 126, a)
are highly characteristic, as are the slender toothed branches
of _Glauconome_ (fig. 126, b). A more singular form, 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_.
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