12 



ELEMENTS OF MAMMALIAN ANATOMY.. 



number of teeth in the different genera varies considerably, 

 as seen by the following formula: 



m.f = 32 



The elephant presents a very specialized dentition. It 

 has no canines nor any lower incisors. The single pair of 

 upper incisors is developed into long tusks, much prized for 

 ivory. They continue to grow throughout the entire life 

 of the animal. Six molars are present on each side, only 

 one or two of which are functional at once. The posterior 

 ones move forward to take the place of the anterior as these 

 become worn out. The tusks of the walrus are its canines 

 greatly developed. None of the typical modern Ruminants 

 have upper incisors in the adult state, though they are 

 present in the embryo. Palaeontology has demonstrated 

 that the ancestors of the Ruminants had well-developed 

 upper incisors in the later Eocene times. The occurrence 

 of the rudimentary upper incisors in the embryo furnishes 

 evidence of a full dentition in the ancestors of the rumi- 

 nants, since the individual embryological history is some- 

 what of a recapitulation of the ancestral history of the race. 



THE PHARYNX. 



The pharynx is that portion of the alimentary canal be- 

 tween the mouth and the esophagus. The structure is well 

 shown by making a sagittal section through the head and 

 neck. This is best done with a saw, after the specimen is 

 frozen by placing it out-of-doors during one or two days 

 of cold winter weather. 



