518 CETACEA. 



tlie dimensions of the former, are only very slightly larger. It may be that this 

 difference in size may have no structural or functional significance ; but it should 

 be borne in mind that the habits of the two forms are distinct, the smaller animal 

 occurring near the coast and frequenting the muddy water of estuaries, and the 

 larger whale living in the clear open sea. It will be further observed, however, 

 that the canals of Monodon, although the animal is smaller than GlobicepJialtis 

 deductor, has slightly larger canals, and therefore still larger than those of Orcella, 

 but, according to the proportional size of the animal, probably relatively smaller 

 than those of Orcella. 



I must again revert to the eye of Platanista, which I have shown to be rudi- 

 mentary, and, in connection with this, call attention to the great development of 

 the semicii'cular canals, as compared with other Cetacean genera possessing well 

 developed eyes, because it has been pointed out by Crum Brown that, when a 

 person is laid flat on a horizontal rotathig table with the eyes closed, every muscle 

 at perfect rest, and thus aU the ordinary means of ascertaining the position of the 

 body removed, the person is still able to form a tolerably accurate judgment as to 

 the angle through which his body is moved, and, according to Crum Brown,^ he 

 does this by means of the sensation instituted by the pressure on the ampullar 

 In'ought about by the movement of the body. He further supposes that the effect is 

 different according as the flow is from the ampulla into the canal or from the canal 

 into the ampulla, and that thus we are able to recognize the direction of the rota- 

 tion, whether positive or negative, ex. gr., to the right or to the left, and so on. Hence 

 the existence of six ampuUse, two for each of the three axes of rotation ; and 

 Crum Brown asserts that in man and all animals which he has examined, as in 

 these Cetacean labyrinths, the two exterior canals of the two ears are very nearly 

 in the same plane, and the superior canal of the one ear very nearly in the same 

 plane as the posterior canal of the other. 'Platanista being practically blind, 

 and having its semicircular canals largely developed, the supposition is permissible, 

 in view of Crum Brown's theory, that the great development may somehow com- 

 pensate for defective vision. 



I must, however, further remark that this development of the canals does 

 3iot appear to be associated with a keen sense of hearing (unless the animal 

 on which I experimented was deaf), because I fired a pistol over it close to its 

 ear, on two occasions, without its exhibiting any sign of having received any 

 impression. 



Tympanic (figs. 8, 9, and 10, PI. XL).— It is a pointedly conical buDate 

 bone lying below the periotic, with its sharp tubular apex directed forwards and 

 inwards between the external and nasal plates of the pterygoid, impinging on the 

 posterior nares in the angle formed by their external and posterior walls. Erom 

 the Eustachian extremity, the bone is traversed longitudinally, in its outer half, by a 

 wide and deep cavity, which terminates posteriorly in a semi-cii'cular expansion of the 



' Forster, Phjs., p. 495. 



