426 THE INHABITANTS OF THE SEA. 
indefatigable zeal of the geologists has discovered no less than 
thirty-nine distinct fossiliferous strata of different ages, and as 
many of these are again subdivided into successive layers, fre 
queutly of a thickness of several thousand feet, and each of them 
characterised by its peculiar organic remains, we may form some 
idea of the vast spaces of time required for their formation. 
The annals of the human race speak of the rise and downfall 
of nations and dynasties, and stamp a couple of thousand years 
with the mark of high antiquity ; but each stratum or each leaf 
in the records of our globe has witnessed the birth and the ex- 
tinction of numerous families, genera, and species of plants and 
animals, and shows us organic Nature as changeable in time as 
she appears to us in space. As, when we sail to the southern 
hemisphere, the stars of the northern firmament gradually sink 
below the horizon, until finally entirely new constellations blaze 
upon us from the nightly heavens; thus in the organic vestiges 
of the paleozoic seas we find no form of life resembling those 
of the actual times, but every class 
“Seems to have undergone a change 
Into something new and strange.” 
Then spiral-armed Brachiopods were the chief representatives 
of the molluses; then crinoid starfishes paved the bottom of 
the ocean; then the fishes, covered with large thick rhon.boidal 
scales, were buckler-headed like the Cephalaspis, or furnished with 
wing-like appendages like the Pterichthys; and then the Tri- 
lobites, a crustacean tribe, thus named from its three lobed 
skeleton, swarmed in the shallow littoral 
waters where the lesser sea-fry afforded 
them an abundant food. From a com- 
parison of their strueture with recent 
analogies, it is supposed that these strange 
creatures swam in an inverted position 
close beneath the surface of the water, the 
belly upwards, and that they made use of 
their power of rolling themselves into a 
ball as a defence against attacks from 
above. The remains of seventeen families 
of Trilobites, including forty-five genera 
Tailobite- and 477 species, some of the size of a pea, 
cthers two feet leng, testify the once flourishing condition of 
