VERTEBRATA. MAMMALIA. 



usual form, strictly Mammalia: their blood is hot, 

 their respiration aerial, their ears open outwardly, 

 their young are produced alive, and suckled after 

 birth with true milk. 



The limbs are greatly modified ; the fore pair are 

 shortened and altered into broad fins, covered with 

 continuous skin, and shewing no division of toes, nor, 

 with one single exception, any claws : the bones of 

 the hand, however, are very distinctly recognized in 

 the skeleton. In some species, there are a few 

 minute bones imbedded in the flesh, unconnected 

 with the spine, which represent the pelvis, but in 

 general, the hind limbs are altogether wanting. The 

 tail is dilated into a broad, flat, double fin, pos- 

 sessed of enormous muscular power, which being 

 placed horizontally, strikes the water upwards and 

 dowrrvvards, and not sidewise as in Fishes. The 

 object of this position and movement is to enable the 

 animal to mount to the surface of the water from 

 great depths, when breathing becomes necessary. 



The vast pressure of so great a volume of water 

 as that to which the Cetacea are often exposed, 

 amounting sometimes to a ton upon each square 

 inch, would crush an ordinary animal instantly to 

 death ; but this result is obviated by a most admir- 

 able contrivance. The blubber, which, as is well- 

 known, invests the body of the Whales to a great 

 thickness, is not, as supposed, beneath the skin, but 

 actually in it. The true skin is found to be con- 

 tinued down to the muscles, of a very open texture, 

 caused by the loose interlacing of numerous fibres, 



