he little spiral discs of _Spirorbis
Lewisii_.

[Illustration: Fig. 64.--Upper Silurian Trilobites. a, _Cheirurus
bimucronatus_, Wenlock and Caradoc; b, _Phacops longicaudatus_,
Wenlock, Britain, and America; c, _Phacops Downingioe_, Wenlock
and Ludlow; d, _Harpes ungula_, Upper Silurian, Bohemia. (After
Salter and Barrande.)]

Amongst the _Articulates_, we still meet only with the remains of
_Crustaceans_. Besides the little bivalved _Ostracoda_--which here
are occasionally found of the size of beans--and various _Phyllopods_
of different kinds, we have an abundance of _Trilobites_. These
last-mentioned ancient types, however, are now beginning to show
signs of decadence; and though still individually numerous, there
is a great diminution in the number of generic types. Many of
the old genera, which flourished so abundantly in Lower Silurian
seas, have now died out; and the group is represented chiefly
by species of _Cheirurus, Encrinurus, Harpes, Proetus, Lichas,
Acidaspis, Illoenus, Calymene, Homalonotus_, and _Phacops_--the
last of these, one of the highest and most beautiful of the groups
of Trilobites, attaining here its maximum of development. In the
annexed illustration (fig. 64) some of the characteristic Upper
Silurian Trilobites are represented--all, however, belonging
to genera which have their commencement in the Lower Silurian
period. In addition to the above, the Ludlow rocks of Britain
and the Lower Helderberg beds of North America have yielded the
remains of certain singular Crustaceans belonging to the extinct
order of the _Eurypterida_. Some of these wonderful forms are
not remarkable for their size; but others, such as _Pterygotus
Anglicus_ (fig. 65), attain a length of six feet or more, and
may fairly be considered as the giants of their class. The
Eurypterids are most nearly allied to the existing King-crabs
(_Limuli_), and have the anterior end of the body covered with
a great head-shield, carrying two pairs of eyes, the one simple
and the other compound. The feelers are converted into pincers,
whilst the last pair of limbs have their bases covered with spiny
teeth so as to act as jaws, and are flattened and widened out
towards their extremities so as to officiate as swimming-paddles.
The hinder extremity of the body is composed of thirteen rings,
which have no legs attached to them; and the last segment of
the tail is either a flattened plate or a narrow, sword-shaped
spine. Fragments of the