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, but few or no
new ones make their appearance during this period in either the
Old World or the New.
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