rms
(_Obolus, Lingulella, Acrotreta, Discinopsis_, &c.); amongst the
articulate genera are _Kutorgina, Orthis, Khynchonella._ It is a
striking fact that certain of these non-articulate "lamp-shells" are
familiar inhabitants of our present seas. Each of the principal groups
of true mollusca was represented: Pelecypods (_Modioloides_);
Gasteropods (_Scenella, Pleurotomaria, Trochonema_); Pteropods
(_Hyohthellus, Hyolithes, Salleretta_); Cephalopods (_Orthoceras,
Cystoceras_). Of land plants no traces have yet been discovered. Certain
markings on slates and sandstones, such as the "fucoids" of Scandinavia
and Scotland, the _Phycoides_ of the Fichtelgebirge, _Eophyton_ and
other seaweed-like impressions, may indeed be the casts of fucoid
plants; but it is by no means sure that many of them are not mere
inorganic imitative markings or the tracks or casts of worms.
_Oldhamia_, a delicate branching body, abundant in the Cambrian of the
south-east of Ireland, is probably a calcareous alga, but its precise
nature has not been satisfactorily determined.

_Cambrian Stratigraphy._--Wherever the Cambrian strata have been
carefully studied it has now been found possible and convenient to
arrange them into three series, each of which is characterized by a
distinctive genus of trilobite. Thus we have a Lower Cambrian with
_Olenellus_, a middle series with _Paradoxides_ and an Upper Cambrian
with _Olenus_. It is true that these fossils are not invariably present
in every occurrence of Cambrian strata, but this fact notwithstanding,
the threefold division holds with sufficient constancy. An uppermost
series lies above the _Olenus_ fauna in some areas; it is represented by
the Tremadoc beds in Britain or by the _Dictyonema_ beds or
_Euloma-Niobe_ fauna elsewhere. Three regions deserve special attention:
(1) Great Britain, the area in which the Cambrian was first
differentiated from the old "Transition Series"; (2) North America, on
account of the wide-spread occurrence of the rocks and the abundance and
perfection of the fossils; and (3) Bohemia, made classic by the great
labours of J. Barrande.

_Great Britain and Ireland._--The table on p. 88 contains the names
that have been applied to the subdivisions of the Cambrian strata in
the areas of outcrop in Wales and England; at the same time it
indicates approximately their relative position in the system.

In _Scotland_ the upper and middle series are represented by a thick