n as "horns," from their shape. Of the many
forms of these, the species of _Cyathophyllum, Heliophyllum_
(fig. 82), _Zaphrentis_ (fig. 81), and _Cystiphyllum_ (fig. 80),
are perhaps those most abundantly represented--none of these
genera, however, except _Heliophyllum_, being peculiar to the
Devonian period. There are also numerous compound Rugose corals,
such as species of _Eridophyllum, Diphyphyllum, Syringopora,
Phillipsastroea_, and some of the forms of _Cyathophyllum_ and
_Crepidophyllum_ (fig. 83). Some of these compound corals attain
a very large size, and form of themselves regular beds, which
have an analogy, at any rate, with existing coral-reefs, though
there are grounds for believing that these ancient types differed
from the modern reef-builders in being inhabitants of deep water.
The "Tabulate Corals" are hardly less abundant in the Devonian
rocks than the _Rugosa_; and being invariably compound, they
hardly yield to the latter in the dimensions of the aggregations
which they sometimes form.

[Illustration: Fig. 84.--Portion of a mass of _Favosites
Gothlandica_, of the natural size. Upper Silurian and Devonian
of Europe and America. (Original.) Billings.]

[Illustration: Fig. 85.--Fragment of _Favosites hemispherica_,
of the natural size. Upper Silurian and Devonian of America.
(After Billings.)]

The commonest, and at the same time the largest, of these are
the "honeycomb corals," forming the genus _Favosites_ (figs.
84, 85), which derive both their vernacular and their technical
names from their great likeness to masses of petrified honeycomb.
The most abundant species are _Favosites Gothlandica_ and _F.
Hemispherica_, both here figured, which form masses sometimes
not less than two or three feet in diameter. Whilst _Favosites_
has acquired a popular name by its honey-combed appearance, the
resemblance of _Michelinia_ to a fossilised wasp's nest with the
comb exposed is hardly less striking, and has earned for it a
similar recognition from the non-scientific public. In addition
to these, there are numerous branching or plant-like Tabulate
Corals, often of the most graceful form, which are distinctive
of the Devonian in all parts of the world.

The _Echinoderms_ of the Devonian period call for little special
notice. Many of the Devonian limestones are "crinoidal;" and
the _Crinoids_ are the most abundant and widely-distributed
representatives of their class in the deposits of this period.

The _C