WHALEBONE WHALES. 
7 
specialised than the other group of Cetaceans; this laxity of connection between 
the ribs and the other parts of the skeleton, allowing of a greater degree of 
expansion of the cavity of the chest, and thus permitting a longer submergence 
beneath the water without the necessity of coming up to breathe. On the other 
hand, in the retention of the double apertures to the nostrils, and in their 
symmetrically-formed skulls, as well as in the position of a distinct olfactory organ, 
and certain other features of their organisation, the members of the present group 
depart less widely from the ordinary type of mammalian structure than do the 
existing toothed Cetaceans. 
^ The baleen or whalebone, which does not appear until after 
birth, is composed of a numerous series of flattened horny plates, 
placed transversely on either side of the palate, and separated from one another by 
an open space in the middle line. The plates vary greatly in length in the different 
species, and are triangular in form, with their broad bases attached to the palate, 
and their points hanging downwards. Although smooth externally, the inner 
edge of each plate is frayed out into a kind of fringe, thus giving a hairy appear¬ 
ance to the whole of the inside of the mouth when viewed from below. The plates 
attain the greatest length in the middle portion of the series, from whence they 
gradually diminish in size towards the two extremities. Baleen differs greatly not 
only in length, but likewise in its relative thickness and degree of elasticity, in the 
