ii;6 THE CETACEANS 



single tooth. In the baleen, or whalebone whales, the place of teeth is taken by the 

 horny structure technically termed "baleen," but commonly known as "whalebone," 

 the nature of which will be explained later on. Even in this group, however, there are 

 rudiments of teeth deeply buried in the gems of the young; and the structure of these 

 rudiments is such as to indicate the origin of at least that group of Cetaceans from 

 Mammals furnished with teeth of a complex type. It is further inferred by Dr. 

 Kiikenthal that these rudimentary teeth correspond with those of the permanent set 

 in other Mammals. 



In conformity with the absence of any external indications of a neck, 

 Neck 



the vertebrae in that region of the backbone of Cetaceans are abnor- 

 mally shortened, so that even in the largest species this part of the column may not 

 much exceed a foot in length. This shortening of the neck is, however, not accom- 

 plished by any reduction in the number of the vertebrae from the normal seven, but by 

 the shortening of the body of each of these vertebrae until it assumes the form of a 

 broad, thin plate. The necks of a giraffe and a whale present, therefore, the ex- 

 tremes in the modifications assumed by their constituent vertebrae; these joints under- 

 going the maximum degree of elongation in the one, and of abbreviation in the other. 

 In many Cetaceans the whole or a certain number of the vertebrae of the neck are 

 welded together into one solid mass. In the hinder portion of the backbone the 

 region of the tail is only distinguished from that of the trunk by the vertebrae (as 

 shown in our figure of the skeleton of the Greenland whale) carrying chevron bones 

 affixed to their under surfaces; there being no mass of united vertebrae, correspond- 

 ing to the sacrum of other Mammals. 



The skeleton of the fore-limbs exhibits all the segments characteriz- 

 ing those of terrestrial Mammals, although some of these are much 

 modified. There are no collar bones (clavicles) ; but the shoulder blades, or scapulae, 

 are very large, and are much elongated in the antero-posterior direction. The 

 humerus, or bone of the upper arm, although much shorter than usual, has a free 

 movement at its articulation with the shoulder blade; but its junction with the two 

 bones of the fore-arm, like all the joints lower down in the limb, admits of scarcely 

 any motion. Indeed, in all these joints the bones articulate by flattened surfaces 

 closely applied to one another, and are bound together by fibrous tissue. The bones 

 of the fore-arm (radius and ulna) are elongated and flattened, and lie nearly parallel, 

 one in front of the other. The number of digits in the flippers is usually five, al- 

 though occasionally reduced to four, and in the second and third of these there are 

 always more than the ordinary three joints below the metacarpus. The only rudi- 

 ments of the hind-limbs are a few small bones beneath the sacral region of the back- 

 bone representing part of the pelvis, and occasionally part of the limb itself (/"and 

 P in the figure on p. 1174). 



In accordance with the position of the nostrils at its summit, the 

 skull departs considerably from the ordinary type. It will suffice to 

 state here that the supraoccipital bone extends forward to join the frontals, and thus 

 excludes the parietal bones from taking any share in the formation of the roof of the 

 middle line of the skull ; while in front of the opening for the nostrils there is a more 

 or less prolonged rostrum. 



