58(> 



ZOOLOGY 



SECT. 



podidse) respectively, the spinal column and limbs allying them 

 with the former, and the crania and the teeth with the latter. 



The Cetacea are represented in the Tertiary (Eocene and 

 Miocene) of Europe, Egypt, and North America by an extinct 

 sub-order Archceoceti or Zeuglodonta, comprising only one known 

 genus Zeuglodon. Zeuglodon differs from existing Cetacea mainly 

 in the possession of rooted heterodont teeth, and in the position of 

 the narial aperture, which is situated comparatively far forwards ; 

 the limbs are not known ; there were irregular dermal bony plates. 

 The remains of both Whalebone Whales and Toothed Whales occur 

 abundantly in Pliocene deposits, some belonging to extinct, others 

 to existing genera. Toothed Whales occur also in Miocene forma- 

 tions, and there, as well as in the Pliocene and Pleistocene of Europe, 

 North America, New Zealand, and Australia, are represented by an 



extinct family, the 

 Squalodontidce, 

 (Fig. 1246), with 

 heterodont denti- 

 tion. 



The order Si- 

 renia is first met 

 with in the Eocene, 

 and was repre- 

 sented in that and 

 succeeding periods 

 by several extinct 

 genera, of which 

 Halitherium is the best known. These were characterised 

 by the possession of upper incisors, in some cases of canines, of 

 enamelled pre-molars and molars, of a milk-dentition, and of small 

 vestiges of femora. The family of the Dugongs is represented by 

 a form nearly allied to the existing genus in the Pliocene of France, 

 and probably by another genus in the Tertiary of California. The 

 family of the Manatees is not known to be represented by any 

 fossil forms. The recently extinct Rhytina (" Steller's Sea-Cow "), 

 which lived within historic times in Behring's Straits, was the 

 largest known member of the order, and sometimes attained a 

 length of seven or eight metres. 



The Tertiary Ungulata comprise an immense number of forms, 

 including numerous extinct families, into an account of which 

 it would be going beyond the scope of the present work to enter. 

 In the Artiodactyle series there is to be traced a progressive union 

 and coalescence of the third and fourth metacarpals to form the 

 cannon-bone, a progressive reduction of the lateral digits, and a 

 progressive development of horns or of tusks absent or rudi- 

 mentary in the earlier representatives of the sub-order. In the 

 Perissodactyle series the reduction of the lateral toes reaches its 



FIG. 1246. Squalodon. Three of the lower true molars. 

 (After Flower.) 



