136 SKETCH OF THE HISTORY OF MAMMALIA. 



cut surface of one of these teeth, each part exhibiting a 

 central pulp-cavity, and concentric striae of growth. For 

 minute details see ' Mag". Nat. Hist.' May, 1839, p. 

 209. 



Order— CETACEA. 



The Cetacea are characterized by the conical fish-like 

 form of the body, covered with a smooth naked skin, 

 adapted for the medium in which they live; between 

 the skin and the muscles is a layer of blubber, varying 

 in thickness in different species, and most abundant in 

 the Greenland whale, its thickness being from eight or 

 ten to twenty inches. The uses of this layer are various : 

 in the first place, it renders the huge body of these ani- 

 mals specifically lighter than the surrounding fluid ; 

 secondly, it materially tends to the preservation of the 

 vital heat ; and, thirdly, it affords protection to the internal 

 organs against the effects of the enormous pressure to 

 which these animals are subject while plunging deep 

 into the abysses of the ocean. The fore limbs are 

 modified into flippers, and the posterior part of the body, 

 destitute of hind limi)s, is continued conical, and termi- 

 nates in a broad horizontal cartilaginous paddle. On 

 the central line of the upper surface, and generally 

 towards the tail, is sometimes seated a small vertical car- 

 tilaginous fin, unconnected with the skeleton. This fin 

 varies in figure, and is often absent. It is partly by the 

 aid of this fin, where present, but more so by that of the 

 flippers, that the Cetacea balance themselves in the water, 

 for when dead or dying they float on the side or the back. 

 The mode of progression consists of a series of leaps or 

 impulses produced by the action of the tail, which is bent 

 down and then struck out with greater or less violence, 

 according to the rapidity w ith which the animal is cleav- 

 ing its way. By means of this organ it can dive instan- 

 taneously, or even leap out of the water, throwing the 

 waves around into sprays and foam. In the ordinary 

 position of the Cetacea while floating, only the top of 

 the head and part of the back appear above the surface, 



