xii.] CETACEA. 219 



right. In consequence of the small increase in the size of 

 the brain-cavity, compared with that of the external parts of 

 the head in these enormous animals, the foramina through 

 which the nerves pass out of the lateral parts of the base 

 of the skull, are long channels excavated through immense 

 bony masses. The petro-tympanic bone, which is scarcely 

 larger than that of some of the small Dolphins, is situated 

 at the bottom of such a channel, at the distance of fourteen 

 inches from the inner wall of the brain cavity. In the 

 general principle of their conformation, these bones do not 

 differ from those of the ordinary Dolphins, but the tongue- 

 shaped backward projection before described is greatly 

 elongated and laminated, being composed of a large num- 

 ber of distinct thin plates, only held together by their com- 

 mon attachment to the tympanic. These fit into grooves 

 between the squamosal and exoccipital, their extremities 

 appearing on the outer surface of the skull, and they serve 

 to attach the petro-tympanic more firmly to the cranium 

 than is the case in the other Toothed Whales. 



The hyoid in Physeter and in the allied genus Kogia is 

 remarkable for the great breadth and flatness of the basi- 

 and the thyro-hyals, which, moreover, do not usually become 

 ankylosed, as in most Dolphins. 



The cranium of the Whalebone Whales (sub-order Mysta- 

 coceti] never shows that deviation from bilateral symmetry 

 so frequent in the Toothed ( Whales. The cranial cavity 

 has much the same general form, and the bones around are 

 disposed in a somewhat similar manner, but the parietals 

 meet at the top of the skull, although completely overlaid 

 and concealed externally by the great supraoccipital. 



The anterior nares are not directed upwards and back- 

 wards as in the Dolphins, but approach more in position to 

 those of the ordinary Mammalia, being arched over by the 



