4 CETACEANS. 



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 gums 



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 vertebras in that region of the backbone of Cetaceans are 



abnormally 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 accomplished by an}- reduction in the number of the vertebra? from the normal 



seven, but by the shortening of the body of each of these vertebra? until it assumes 



the form of a broad, thin plate. The necks of a giraffe and a whale present, 



therefore, the extremes in the modifications assumed by their constituent vertebra 1 ; 



these joints undergoing 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 vertebra? 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-surf aces ; there being no 



mass of united vertebra?, corresponding to the sacrum of other mammals. 



The skeleton of the fore-limbs exhibits all the segments 

 Limbs. . . ° 



characterising those of terrestrial mammals, although some of these 



are much modified There are no collar-bones (clavicles) ; but the shoulder-blades, 



or scapula?, 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 bo :es 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 b} T 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 Ave, although 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 rudiments of the hind-limbs are a few small bones beneath the sacral 



region of the backbone representing part of the pelvis, and occasionally part of the 



limb itself (/ and p. in the figure on p. 2). 



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



Skull . 



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



state here that the supraoccipital bone extends forward to join the f rentals, 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. 



In regard to the soft parts, it may be mentioned that the stomach 

 Soft Parts. . r 



is always complex : and that the female has two teats, placed far 



back on the abdomen. In order to enable these animals to swim with their mouths 



