ystideans_, with doubtful exceptions, have not been recognised
in the Devonian; and their place is taken by the allied group of
the "Pentremites," which will be further spoken of as occurring
in the Carboniferous rocks. On the other hand, the Star-fishes,
Brittle-stars, and Sea-urchins are all continued by types more
or less closely allied to those of the preceding Upper Silurian.

Of the remains of Ringed-worms (_Annelides_), the most numerous
and the most interesting are the calcareous envelopes of some
small tube-inhabiting species. No one who has visited the seaside
can have failed to notice the little spiral tubes of the existing
_Spirorbis_ growing attached to shells, or covering the fronds
of the commoner Sea weeds (especially _Fucus serratus_). These
tubes are inhabited by a small Annelide, and structures of a
similar character occur not uncommonly from the Upper Silurian
upwards. In the Devonian rocks, _Spirorbis_ is an extremely common
fossil, growing in hundreds attached to the outer surface of
corals and shells, and appearing in many specific forms (figs.
86 and 87); but almost all the known examples are of small size,
and are liable to escape a cursory examination.

[Illustration: Fig. 87.--a, _Spirobois omphalodes_, natural size
and enlarged. Devonian, Europe and America; b, _Spirorbis
Arkonensis_, of the natural size and enlarged; c, The same,
with the tube twisted in the reverse direction. Devonian, America.
(Onginal.)]

[Illustration: Fig. 88. a b, _Spirorbis laxus_, enlarged, Upper
Silurian, America; c, _Spirorbis spinulifera_, of the natural
size and enlarged, Devonian, Canada. (After Hall and the Author.)]

[Illustration: Fig. 88.--Devonian Trilobites; a, _Phacops latifrons_,
Devonian of Britain, the Continent of Europe, and South America;
b, _Homalonotus armatus_, Europe; c, _Phacops (Trimerocephalus)
loevis_, Europe; d, Head-shield of _Phacops (Portlockia)
granulatus_, Europe. (After Salter and Burmeister.)]

The _Crustaceans_ of the Devonian are principally _Eurypterids_
and _Trilobites_. Some of the former attain gigantic dimensions,
and the quarrymen in the Scotch Old Red give them the name of
"seraphim" from their singular scale-like ornamentation. The
_Trilobites_, though still sufficiently abundant in some localites,
have undergone a yet further diminution since the close of the
Upper Silurian. In both America and Europe quite a number of
generic types have survived from the Silurian,