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_).
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