426 THE VERTEBRATE SKELETON. 



CETACEA. 



ARCHAEOCETI. Zeuglodon has the following dentition, 



31 5 



i ^ c - pm and m -=, total 36. The incisors and canines are 

 31 5 



simple and conical ; the cheek teeth are compressed and have 

 serrated cutting edges like those in some seals. 



In the MYSTACOCETI, or whalebone whales, calcified tooth 

 germs probably belonging to the milk dentition are present 

 in the embryo, but they are never functional, and are al- 

 together absent in the adult. The anterior of these germs 

 are simple, the posterior ones are originally complex, but 

 subsequently split up into simple teeth like those of the 

 anterior part of the jaw. Hence according to Kukenthal, 

 who described these structures, the Cetacean dentition was 

 originally heterodont. 



In the living ODONTOCETI the dentition is homodont and 

 monophyodont. In some cases traces occur of a replacing 

 dentition which never comes to maturity, and renders it pro- 

 bable that the functional teeth of the Odontoceti are really 

 homologous with the milk teeth of other mammals. Some of 

 the dolphins afford the apparently simplest type of mammalian 

 dentition known. The teeth are all simple, conical, slightly 

 recurved structures, with simple tapering roots and without 

 enamel. The dentition is typically piscivorous, being adapted 

 for seizing active slippery animals such as fish. The prey is 

 then swallowed entire without mastication. Sometimes the 

 teeth are excessively numerous, reaching two hundred or more 

 (fifty to sixty on each side of each jaw) in Pontoporia. This 

 multiplication of teeth is regarded by Kukenthal as due to the 

 division into three parts of numbers of trilobed teeth similar to 

 those of some seals. 



In the Sperm whale, Physeter, the lower jaw bears a series 

 of twenty to twenty-five stout conical recurved teeth, while in 

 the upper jaw the teeth are vestigial and remain imbedded in 

 the gum. An extinct form, Physodon, from the Pliocene of 



