xv DENTITION 215 



Odontocetes the olfactory apparatus, as well as that part of 

 the brain specially related to the function of smell, is entirely 

 wanting, but in neither group is there the slightest trace of 

 the specially aquatic olfactory organ of fishes. Its complete 

 absence and the presence of vestiges of the aerial organ of land 

 mammals in the Mystacocetes are the clearest possible indi- 

 cations of the origin of the Cetacea from air-breathing and air- 

 smelling terrestrial mammalia. With their adaptation to an 

 aquatic mode of existence, organs fitted only for smelling in 

 air became useless, and so have dwindled or completely dis- 

 appeared. Time and circumstances do not seem to have per- 

 mitted the acquisition of anything analogous to the specially 

 aquatic smelling apparatus of fishes, the result being that 

 whales are practically deprived of whatever advantage this 

 sense may be to other animals. 



It is characteristic of the greater number of mammalia to 

 have their jaws furnished with teeth having a definite 

 structure and mode of development. In all the most typical 

 forms these teeth are limited in number, not exceeding 

 eleven on each side of each jaw, or forty-four in all, and are 

 differentiated in shape in different parts of the series, being 

 more simple in front, broader and more complex behind. Such 

 a dentition is described as " heterodont." In most cases also 

 there are two distinct sets of teeth during the lifetime of the 

 animal, constituting a condition technically called "diphyodont." 



All the Cetacea present some traces of teeth, which in 

 structure and mode of development resemble those of mammals, 

 and not those of the lower vertebrated classes, but they are 

 always found in a more or less imperfect state. In the first 

 place, at all events in existing species, they are never truly 

 heterodont, all the teeth of the series resembling each other 

 more or less, or belonging to the condition called " homodont," 

 and not obeying the usual numerical rule, often falling short 

 of, but in many cases greatly exceeding it. The most typical 

 Odontocetes, or toothed whales, have a large number of 

 similar, simple, conical, recurved, pointed teeth, alike on both 

 sides and in the upper and under jaws, admirably adapted 

 for catching slippery, living prey, such as fish, which 

 are swallowed whole without mastication. In one genus 



