408 ELEMENTARY GEOLOGY 
characteristic groups of Cambrian fossils are the trilo- 
bites (Plate 16) and the brachiopods.. Of these, the 
former have since become extinct, and hence are not 
represented in the modern ocean; but the latter, 
although greatly reduced in numbers and variety, 
are still found in various parts of the sea. It is im- 
possible to predict what the ancestry of these creatures 
was. 
Life in the Ordovician. — During the next age, the 
Ordovician, or Lower Silurian, the life characteristics 
are very similar to those of the Cambrian. In this 
period the trilobites attained their greatest develop- 
ment; but after the Ordovician, these interesting 
creatures began to decline in numbers and variety, 
until their final extinction near the close of the Paleo- 
zoic. Brachiopods (Fig. 255) have also developed ex- 
tensively, but have not yet reached their height of 
development. 
While the Cambrian animal life of the sea is ex- 
clusively that of invertebrates, recent discoveries in 
Colorado possibly indicate that the higher forms of 
vertebrate life began to compete with invertebrates 
for dominion in the Ordovician sea. Fossils of true 
fishes are reported from the Ordovician rocks of Colo- 
rado; but this discovery is not universally accepted, 
and there is still doubt whether fishes really did 
appear before the next period, the Silurian. 
