356 THE VERTEBRATE SKELETON. 



Suborder (1). ARCHAEOCETI. 



The members of this group are extinct; they differ from 

 all living Cetacea in having the dentition heterodont and in 

 the fact that the back was probably protected by dermal 

 plates. The skull is elongated and depressed, and the brain 

 cavity is very small. The temporal fossae are large, and 

 there is a strong sagittal crest. The nasals and premaxillae 

 are a good deal larger than they are in living Cetacea, and 

 the anterior nares are usually far forward. The cervical ver- 

 tebrae are not fused with one another, and the lumbar 

 vertebrae are unusually elongated. 



The limbs are very imperfectly known, but while the 

 humerus is much longer than in modern Cetaceans, it is 

 nevertheless flattened distally, indicating that the limb was 

 paddle-like, and that there was scarcely any free movement 

 between the fore-arm and upper arm. 



The best known genus is Zeuglodon, which is found in 

 beds of Eocene age in various parts of Europe, and in Alabama. 



Suborder (2). MYSTACOCETI or BALAENOIDEA. 



These are the Whalebone Whales or True Whales. 



Calcified teeth representing the milk dentition occur in the 

 foetus, but the teeth are never functional, and always dis- 

 appear before the close of foetal life. There is a definite 

 though small olfactory fossa. The palate is provided with 

 plates of baleen or whalebone. The skull is symmetrical, and 

 is extremely large in proportion to the body. The nasals are 

 moderately well developed, and the maxillae do not overlap 

 the orbital processes of the f rentals. The lachrymals are small 

 and distinct from the jugals. The tympanics are ankylosed 

 to the periotics, and the rami of the mandible do not meet in 

 a true symphysis. The ribs articulate only with the transverse 

 processes, and the capitula are absent or imperfectly deve- 

 loped. Only one pair of ribs meets the sternum, which is 

 composed of a single piece. 



