crocheilina_, _Euomphalus_. Among the pteropods, _Tentaculites_ was
very abundant in some quarters; others were _Conularia_ and
_Styliolina_. In the Devonian period the cephalopods began to make a
distinct advance in numbers, and in development. The goniatites appear
with the genera _Anarcestes_, _Agoniatites_, _Tornoceras_, _Bactrites_
and others; and in the upper strata the clymenoids, forerunners of the
later ammonoids, began to take definite shape. While several new
nautiloids (_Homaloceras_, _Ryticeras_, &c.) made their appearance
several of the older genera still lived on (_Orthoceras_,
_Poterioceras_, _Actinoceras_).

Crinoids were very abundant in some parts of the Devonian sea, though
they were relatively scarce in others; they include the genera
_Melocrinus_, _Haplocrinus_, _Cupressocrinus_, _Calceocrinus_ and
_Eleuthrocrinus_. The cystideans were falling off (_Proteocystis_,
_Tiaracrinus_), but blastoids were in the ascendant (_Nucleocrinus_,
_Codaster_, &c.). Both brittle-stars, _Ophiura_, _Palaeophiura_,
_Eugaster_, and true starfishes, _Palaeaster_, _Aspidosoma_, were
present, as well as urchins (_Lepidocentrus_).

When we turn to the crustaceans we have to deal with two distinct
assemblages, one purely marine, trilobitic, the other mainly
lacustrine or lagoonal with a eurypteridian facies. The trilobites had
already begun to decline in importance, and as happens not
infrequently with degenerating races of beasts and men, they began to
develop strange eccentricities of ornamentation in some of their
genera. A number of Silurian genera lived on into the Devonian period,
and some gradually developed into new and distinctive forms; such were
_Proetus_, _Harpes_, _Cheirurus_, _Bronteus_ and others. Distinct
species of _Phacops_ mark the Lower and Upper Devonian respectively,
while the genus _Dalmania_ (_Odontochile_) was represented by species
with an almost world-wide range. The Ostracod _Entomis_ (_Cypridina_)
was extremely abundant in places--_Cypridinen-Schiefer_--while the
true _Cypridina_ was also present along with _Beyrichia_,
_Leperditia_, &c. The Phyllocarids, _Echinocaris_, _Eleuthrocaris_,
_Tropidocaris_, are common in the United States. It is in the Old Red
Sandstone that the eurypterids are best preserved; foremost among
these was _Pterygotus_; _P. anglicus_ has been found in Scotland with
a length of nearly 6 ft.; _Eurypter