4S6 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- 

 quently 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 palaeozoic 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 molluscs; then crinoid starfishes paved the bottom of 

 the ocean ; then the fishes, covered with large thick rhomboidal 

 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 structure 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 

 Triiobite. an( j 477 species, some of the size of a pea, 



others two feet long, testify the once flourishing condition of 



