but few or no
new ones make their appearance during this period in either the
Old World or the New. The _species_, however, are distinct; and
the principal forms belong to the genera _Phacops_ (fig. 88, a,
c, d), _Homalonotus_ (fig. 88, b), _Proetus_, and _Bronteus_.
The species figured above under the name of _Phacops latifrons_
(fig. 88, a), has an almost world-wide distribution, being found
in the Devonian of Britain, Belgium, France, Germany, Russia,
Spain, and South America; whilst its place is taken in North
America by the closely-allied _Phacops rana_. In addition to the
_Trilobites_, the Devonian deposits have yielded the remains of a
number of the minute _Ostracoda_, such as _Entomis_ ("_Cypridina_"),
_Leperditia_, &c., which sometimes occur in vast numbers, as
in the so-called "_Cypridina_ Slates" of the German Devonian.
There are also a few forms of _Phyllopods_ (_Estheria_). Taken
as a whole, the Crustacean fauna of the Devonian period presents
many alliances with that of the Upper Silurian, but has only
slight relationships with that of the Lower Carboniferous.

Besides _Crustaceans_, we meet here for the first time with the
remains of _air-breathing Articulates_, in the shape of _Insects_.
So far, these have only been obtained from the Devonian rocks of
North America, and they indicate the existence of at least four
generic types, all more or less allied to the existing May-flies
(_Ephemeridoe_). One of these interesting primitive insects, namely,
_Platephemera antiqua_ (fig. 89), appears to have measured five
inches in expanse of wing; and another (_Xelloneura antiquorum_) has
attached to its wing the remains of a "stridulating-organ" similar
to that possessed by the modern Grasshoppers--the instrument, as
Principal Dawson remarks, of "the first music of living things
that Geology as yet reveals to us."

[Illustration: Fig. 89.--Wing of _Platephemera antiqua_ Devonian,
America. (After Dawson.)]

Amongst the _Mollusca_, the Devonian rocks have yielded a great
number of the remains of Sea-mosses (_Polyzoa_). Some of these
belong to the ancient type _Ptilodictya_, which seems to disappear
here, or to the allied _Clathropora_ (fig. 90), with its fenestrated
and reticulated fronds. We meet also with the graceful and delicate
stems of _Ceriopora_ (fig. 91).

[Illustration: Fig. 90.--Fragment of _Clathropora intertexta_,
of the natural size and enlarged. Devonian, Canada. (Original.)]

[Illustration: Fig