364 Professor Flower [May 25, 



adapted for tlie perception of odorous substances permeating the 

 water; the terminations of the olfactory nerves are spread over 

 the inner plicated surface of a cavity near the front part of the 

 nose, to which the fluid in which the animals swim has free access, 

 although it is quite unconnected with the respiratory passages. 

 Mammals, on the other hand, smell substances with which the 

 atmosphere they breathe is impregnated ; their olfactory nerve is dis- 

 tributed over the more or less complex foldings of the lining of a 

 cavity placed more deeply in the head, but in immediate relation to 

 the passages through which air is continually driven to and fro on its 

 way to the lungs in respiration, and therefore in a most favourable 

 position for receiving imjiressions from substances floating in that air. 

 The whalebone whales have an organ of smell exactly on the mam- 

 malian type, but in a rudimentary condition. The perception of 

 odorous substances diffused in the air, upon which many land mam- 

 mals depend so much for obtaining their food, or for protection from 

 danger, can be of little importance to them. In the more comjjletely 

 modified Odontocetes the olfactory aj)2)aratus, as well as that part of 

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

 but in both grou]3S there is not the slightest trace of the sj)ecially 

 aquatic olfactory organ of fishes. Its comj^lete absence and the 

 vestiges of the aerial organ of land mammals found in the Mystaco- 

 cetes are the clearest j)ossible indications 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 

 disappeared. Time and circumstances have not permitted the acquisi- 

 tion 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 niunber 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. 



