70 THE STORY OF THE EAETH AND MAN. 



the Silurian, and to have there attained to some of 

 their grandest forms. The modern pearly nautilus 

 shell, well known in every museum, is beautifully 

 coiled in a disc-like form, and when sliced longitudi- 

 nally shows a series of partitions dividing it into 

 chambers, air-tight, and serving as a float to render 

 the body of the creature independent of the force of 

 gravity. As the animal grows it retracts its body 

 toward the front of the shell, and forms new par- 

 titions, so that the buoyancy of the float always 

 corresponds with the weight of the animal; while by 

 the expansion and contraction of the body and removal 

 of water from a tube or syphon which traverses the 

 chambers, or the injection of additional water, slight 

 differences can be effected, rendering the creature a 

 very little lighter or heavier than the medium in 

 which it swims. Thus practically delivered from the 

 encumbrance of weight, and furnished with long 

 flexible arms provided with suckers, with great eyes 

 and a horny beak, the nautilus becomes one of the 

 tyrants of the deep, creeping on the bottom or 

 swimming on the surface at will, and everywhere 

 preying on whatever animals it can master. Fortu- 

 nately for us, as well as for the more feeble inhabit- 

 ants of the sea, the nautili are not of great size, 

 though some of their allies, the cuttle-fishes, which, 

 however, want the floating apparatus, are sufficiently 

 powerful to be formidable to -man. In the Silurian 

 period, however, there were not only nautili like ours, 

 but a peculiar kind of straight nautilus the Orthocer* 



