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. Ktikenthal 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, the vertebrae 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 any reduction in the number of the vertebrae from the normal 

 seven, but by the shortening of the body of each of these vertebras 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 vertebrae ; 

 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 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 vertebras (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 vertebrae, corresponding to the sacrum of other mammals. 



The skeleton of the fore-limbs exhibits all the segments 



characterising those of terrestrial mammals, although some of these 

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

 or scapulas, 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, 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 departs considerably from the ordinary type. It will suffice to 

 state here that the supraoccipital bone extends forward to join the front als, 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 



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 



