Devonian, Canada. (Original.)]
[Illustration: Fig. 94.--Fragment of _Fenestella cribrosa_, of
the natural size and enlarged. Dovonian, Canada. (Original.)]
The majority of the Devonian _Polyzoa_ belong, however, to the
great and important Palaeozoic group of the Lace-corals (_Fenestella_,
figs. 92 and 94, _Retepora_, fig. 93, _Polypora_, and their allies).
In all these forms there is a horny skeleton, of a fan-like or
funnel-shaped form, which grew attached by its base to some foreign
body. The frond consists of slightly-diverging or nearly parallel
branches, which are either united by delicate cross-bars, or which
bend alternately from side to side, and become directly united
with one another at short intervals--in either case giving origin
to numerous oval or oblong perforations, which communicate to the
whole plant-like colony a characteristic netted and lace-like
appearance. On one of its surfaces--sometimes the internal, sometimes
the external--the frond carries a number of minute chambers or
"cells," which are generally borne in rows on the branches, and
of which each originally contained a minute animal.
[Illustration: Fig. 95.--_Spirifera sculptilis_. Devonian, Canada.
(After Billings.)]
[Illustration: Fig. 96.--_Spirifera mucronata_. Devonian, America.
(After Billings.)]
[Illustration: Fig. 97.
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