MAMMALIA. CETACEA. 353 



radius and ulna, and these two bones are about equally de- 

 veloped, and are often fused together. There are no clavicles, 

 and the pelvis is vestigial, consisting of a pair of somewhat 

 cylindrical bones suspended at some distance from the ver- 

 tebral column. In living forms there is no trace of a posterior 

 limb, but in Halitherium there is a vestigial femur connected 

 with each half of the pelvis. 



Order 3. CETACEA 1 . 



In these mammals the general form is more fish-like than is 

 the case even in the Sirenia. The skin is generally almost 

 completely naked, but hairs are sometimes present in the neigh- 

 bourhood of the mouth, especially in the foetus. In some 

 Odontoceti vestiges of dermal ossicles have been described, and 

 in Zeuglodon the back was probably protected by dermal plates. 

 The anterior limbs have the form of flattened paddles, showing 

 no trace of nails, the posterior limb bones are quite vestigial 

 or absent, and there is never any external sign of the limb. 

 Teeth are always present at some period of the life history, but 

 in the whalebone whales they are only present during foetal 

 life, their place in the adult animal being taken by horny 

 plates of baleen. In all living forms the teeth are simple 

 and uniform structures without enamel ; they have single 

 roots, and the alveoli in which they are imbedded are often 

 incompletely separated from one another. As in some forms 

 traces of a replacing dentition have been described, it has 

 been concluded that the functional teeth of Cetacea belong to 

 the milk dentition. 



The texture of the bones is spongy. The cervical vertebrae 

 are very short, and though originally seven in number, are in 

 many forms completely fused, forming one solid mass (fig. 67). 

 The odontoid process of the axis is short and blunt, or may 



1 See P. J. van Beneden and P. Gervais, Osteographie des Cetaces, 

 186980. 



K. 23 



