The most abundant forms are the
cylindrical, twisted bodies (Planolites), which are so frequently
found on the surfaces of sandy beds, and which have been described
as the stems of sea-weeds. These fossils (fig. 63), however,
can be nothing more, in most cases, than the filled-up burrows
of marine worms resembling the living Lob-worms. There are also
various remains which belong to the group of the tube-inhabiting
Annelides (_Tubicola_). Of this nature are the tubes of _Serpulites_
and _Cornultites_, and the 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.
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