1306 CLASS MAMMALIA. 



occasionally two pairs, are rudimentary. Hyperoodon is represented 

 in the Crags of England and Belgium. These beds also yield the 

 extinct genus Choneziphius, as well as several species belonging to 

 the living genus Mesoplodon, which is distinguished from Chone- 

 ztphius by the complete ossification in the adult of all the elements 

 comprising the cranial rostrum. The Crag species of Mesoplodon 

 are mainly known by these solid rostra, which in the living species 

 are composed of dense ivory-like bone, and are the most solid bones 

 found in the whole of the Vertebrata. The periotics (fig. 11 76) are 

 more rarely found, but are equally characteristic. 



Family Platanistidte. — This family, which is now represented 

 by Platanista of the Ganges, and Inia and Pontoporia of the great 

 rivers of South America, is characterised by the large number of 

 simple teeth in the elongated jaws, and by the length of the man- 

 dibular symphysis exceeding half that of the entire mandible. None 

 of the cervical vertebrae are anchylosed together. Remains allied 

 to Pontoporia were found by Bravard in the Pleistocene of South 

 America, but these are referred by some authorities to a distinct 

 genus under the name of Pontistes or Palceopontoporia. The extinct 

 genera Champsodelphis and Schizodelphis, of the European Pliocene, 

 are usually referred to this family, but the latter approximates to the 

 Delphinidce. 



Family Squalodontidte. — The extinct Squalodonts were for- 

 merly classed with the Zeuglodontidce, but the characters of the 

 cranium are essentially Delphinoid, although the teeth are differenti- 



Three lower molars of Squalodon ; from the Miocene of Europe. 



ated into groups as in the former. The type genus Squalodon ranges 

 from the Middle Miocene to the Pleistocene of Europe, and is also 

 found in the Tertiaries of North America, New Zealand, and Aus- 

 tralia. The teeth may be arranged as I. -, C. -, Pm. -, M. - ; 



3i 4 7 



the premolars are simple, but the true molars (fig. 11 77) have 

 double roots, and crowns very like those of Zeuglodon, although 



