, three outstanding facts are apparent: (1)
the great divergence between the Cambrian fauna and that of the present
day; (2) the Cambrian life assemblage differs in no marked manner from
that of the succeeding Ordovician and Silurian periods; there is a
certain family likeness which unites all of them; (3) the extraordinary
complexity and diversity not only in the assemblage as a whole but
within certain limited groups of organisms. Although in the Cambrian
strata we have the oldest known fossiliferous rocks--if we leave out of
account the very few and very obscure organic remains hitherto recorded
from the pre-Cambrian--yet we appear to enter suddenly into the presence
of a world richly peopled with a suite of organisms already far advanced
in differentiation; the Cambrian fauna seems to be as far removed from
what must have been the first forms of life, as the living forms of this
remote period are distant from the creatures of to-day.

With the exception of the vertebrates, every one of the great classes of
animals is represented in Cambrian rocks. Simple protozoa appear in the
form of Radiolaria; Lithistid sponges are represented by such forms as
_Archaeoscyphia_, Hexactinellid sponges by _Protospongia_; Graptolites
(_Dictyograptus (Dictyonema)_) come on in the higher parts of the
system. Medusa-like casts have been found in the lower Cambrian of
Scandinavia (_Medusina_) and in the mid-Cambrian of Alabama
(_Brooksella_). Corals, _Archaeocyathus, Spirocyathus_, &c., lived in
the Cambrian seas along with starfishes (_Palaeasterina_), Cystideans,
_Protocystites, Trochocystites_ and possibly Crinoids, _Dendrocrinus_.
Annelids left their traces in burrows and casts on the sea-floor
(_Arenicolites, Cruziana, Scolithus_, &c.). Crustacea occupied an
extremely prominent place; there were Phyllocarids such as
_Hymenocaris_, and Ostracods like _Entomidella_; but by far the most
important in numbers and development were the Trilobites, now extinct,
but in palaeozoic times so abundant. In the Cambrian period trilobites
had already attained their maximum size; some species of _Paradoxides_
were nearly 2 ft. long, but in company with these monsters were tiny
forms like _Agnostus_ and _Microdiscus_. Many of the Cambrian trilobites
appear to have been blind, and they had not at this period developed
that flexibility in the carapace that some forms acquired later.

Brachiopods were fairly abundant, particularly the non-articulated fo