THE SILURIAN STRATA PROPER. 
39 
in some places so distinct as to be easily distinguish¬ 
able. 
At the close of the Cambrian age another change 
in the level of the crust must have occurred. The 
land areas were enlarged, and the sea bottom about 
the existing shores was raised. How much time 
intervened is not known. In Wales the interval 
is more marked than in this country, and it was in 
Siluria, Wales, that Professor Murchison first studied 
this new formation. To distinguish it from the Cam¬ 
brian, which he called the Lower Silurian, he gave it 
the name Upper Silurian. These two rock systems 
have been described in Chapter . 
II as formation Humber 2. 
During this age thick strata 
of limestone were deposited. 
Cup, chain, and honeycomb 
corals abounded, forming great 
reefs everywhere. As ripple 
and other shallow water-marks 
are frequently met in the strata, 
the sea must have been com¬ 
paratively shallow. 
Most of the forms of life in¬ 
troduced in the foregoing age 
continued, while many new and 
distinct species were added. 
More than ten thousand species belonging to this age 
alone have been described. Among the new forms 
we find two striking cephalopods {cephal, head -|- 
pod^ foot). One was a long, straight-chambered sea- 
Fig. 19. — Conularia, a 
mollusk having a py¬ 
ramidal shell. 
