MAMMALIA. 285 



grooved, are imbedded in the front of the mandible. Then comes a 

 diastema 011 either side, succeeded by five grinding teeth, of which two 

 are premolarx, and three molar*. They are similar to the upper grinders, 

 but, except the last, more strongly ridged, and grooved internally as well 

 as externally. 



In many animals four pointed canine teeth are present, two above, 

 immediately behind the pre-maxillo-maxillary suture, and two below. 

 There is no trace of these in the Rabbit. 



It is convenient to express the number and kind of teeth by 

 a dental formula. Each kind is indicated by a fraction (preceded 

 by its initial letter), in which the numerator and denominator 

 signify upper and lower teeth, those of opposite sides being 

 separated by dashes. Thus the Rabbit's dental formula is : 



. _ 2 2 _0-0 _3-3 _ 3 3 _ 9 



~ 1 - 1 ~ - P ' m ' ~ 2~~^~2 m ' ~ 3^-~3 ~ 



Those before the vertical line have predecessors in the milk- 

 dentition. As, however, the teeth of one side only need be ex- 

 pressed, those of the other exactly corresponding, and the kinds 

 are easily remembered, the formula may be simplified to : 



2033 

 1023' 



Two rudimentary incisors have been found in the front of each jaw, 

 which are probably persistent members of the milk dentition. They are 

 not included in the above formulae. 



3 1 42 



The dental formula of the dog is 7777777 The canines are strong, pointed 



teeth, adapted for holding prey. All the premolars and the first lower 

 molars have sharp cutting crowns, which are very large in the last upper 

 premolars and first lower molars. These are known as carnassial teeth, 

 and work against one another like the blades of a pair of scissors. The 

 upper molars and last two lower molars have grinding crowns. 



All the teeth have continuous coveriug of enamel in their crowns, and 

 taper below into pointed " fangs. " They do not continue to grow through- 

 out life as in the rabbit. 



The soft palate hangs down behind, and imperfectly separates 

 the mouth-cavity from the pharynx, a small chamber which 

 passes back into the gullet An unpaired oval opening, the 

 internal naris, opens into it in front, and on its floor, at the root 

 of the tongue, is the glottis, guarded in front by a thin flexible 

 flap, the epiglottis, supported by cartilage. The gullet (oesophagus) 

 is a narrow thick-walled tube, which runs from the pharynx 

 through the neck and thorax, just ventral to the backbone, 



