PRESSURE AT OCEAN DEPTHS. 319 



living, the crocodiles. But ia point of fact the specific gravity 

 of its bones was less, for they are traversed by very large 

 cavities which, Professor Marsh says, have all the appearance of 

 having been air-cavities, and after careful investigation he does 

 not hesitate to pronounce them decidedly to be such. I, in 

 company with him, examined these bones, though not, I must- 

 admit, at any great length, and I confess that from their struc- 

 ture alone it did not seem to me possible to prove that this 

 reptile had actually had pneumatic bones like those of birds. 

 It is, however, possible that the calculation, which must have 

 been intrinsically one of great difficulty, may have been erro- 

 neous, and in that case, in my opinion, the most weighty argu- 

 ment for Professor Marsh's view would disappear. But if in 

 fact his friend's calculation as to the maximum size of a reptile 

 having the specific gravity of the crocodile is correct, I believe 

 also that the large cavities which undoubtedly exist in the 

 bones of that fossil creature can have been nothing else than 

 air-cavities, whose function it was to render the animal light 

 enough for it to carry the still considerable weight inseparable 

 from such an enormous mass. 



The media in which animals live also exert a certain pressure 

 depending on their mass and specific gravity, and it is easy 

 therefore to imagine that all creatures which either fly in the 

 air, swim in water, or creep in mud, must be afieoted by the 

 pressure of the superincumbent mass. This, of itself, is quite 

 true; but this true view has, even in quite recent times, often 

 led to perfectly false issues. The most striking instance of 

 this perverted application of a true idea is offered in the case 

 of animals living at great depths, in fresh as well as in salt 

 water. It used formerly to be said, and the idea is not uncom- 

 monly expressed even at the present day, that it was most won- 

 derful that animals generally, and more particularly such deli- 

 cate structures as Polypes, many Worms, Univalves, &c., were 

 capable of enduring the enormous pressure of the vast volume 

 of water above them, amounting in ocean depths to that of 

 many atmospheres. But, put in this form, the idea is simply 

 absurd ; for the soft-bodied creatures living at the bottom of 

 the sea are no more conscious of the pressure above them of a 



