406 CLASSIFICATION OF VERTEBRATES. 



have come from some more normal quadripedal mammal. IMore 

 recently the view has been gaining ground that the two living 

 divisions, the toothed and the whalebone whales, ma)" have had 

 diverse origins, and their present resemblance maj' be due to 

 convergence rather than to community of descent. The best 

 guesses as to their ancestors would trace them either to the 

 carnivores or to some long-tailed ungulate, while the presence 

 of dermal ossicles in one species of Xeonieris, and possibly in 

 Zeiiglodon, needs to be taken into account. 



The skin is smooth and naked, without hairs ; even the 

 bristles around the mouth may disappear in the adult. There 

 is no neck ; the head is large, and may form one-third of the 

 total length. The eye is small, and without nictitating mem- 

 brane ; the nostrils, separate or with a common crescentic 

 opening, are on the top of the head ; there is no external ear, 

 the small meatus opening close behind the eye. Beneath the 

 skin is the thick layer of fat or ' blubber.' The bones are 

 light and spongy. The cervical vertebrae are more or less 

 completeh' fused, and have the zygopophyses poorly developed. 

 There is no sacrum, but the caudal vertebrae are distinguished 

 from the lumbar by the presence of chevron bones. The 

 sternum is very variable, and is reached by only one (mvs- 

 tacocetes), or a few ribs. The skull consists of a nearly spherical 

 cranium, from which the facial part projects like a beak. In 

 the cranium the roof is formed by the supraoccipital and inter- 

 parietal, which extend forward to meet the frontals, excluding 

 the parietals from the middle line. The frontals are expanded 

 laterally to form roofs to the orbits. The maxillae are verv 

 large, the nasals very small. The lower jaw lacks an ascending 

 ramus. Clavicles are lacking, and there is no elbow joint. 

 The bones of the wrist and hand remain almost entirelv carti- 

 laginous in the whalebone whales. The digits are four or five, 

 the phalanges of the second and third being increased in num- 

 ber up to fourteen, a condition recalling the ichthyosaurs (p. 312). 

 The pelvis is represented by two bones, free from the vertebral 

 column, which, on account of their muscular relations, are 

 regarded as ischia. In some species rudiments of the skeleton 

 of hind limbs occur, imbedded in the flesh. 



