Many of them are extremely small, averaging about
the size of a millet-seed; but a few forms, such as _Entomoconchus
Scouleni_ (fig. 122, c), may attain a length of from one to
three quarters of an inch. The old group of the _Phyllopods_
is is likewise still represented in some abundance, partly by
tailed forms of a shrimp-like appearance, such as _Dithyrocaris_
(fig. 122, d), and partly by the curious striated _Estherioe_
and their allies, which present a curious resemblance to the
true Bivalve Molluscs (fig. 122, b). Lastly, we meet for the
first time in the Carboniferous rocks with the remains of the
highest of all the groups of _Crustaceans_--namely, the so-called
"Decapods," in which there are five pairs of walking-limbs, and
the hinder end of the body ("abdomen") is composed of separate
rings, whilst the anterior end is covered by a head-shield or
"carapace." All the Carboniferous Decapods hitherto discovered
resemble the existing Lobsters, Prawns, and Shrimps (the _Macrura_),
in having a long and well-developed abdomen terminated by an
expanded tail-fin. The _Paloeocaris typus_ (fig. 122, e) and the
_Anthrapaloemon gracilis_ (fig. 122, f), from the Coal-measures
of Illinois, are two of the best understood and most perfectly
preserved of the few known representatives of the "Long-tailed"
Decapods in the Carboniferous series.
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