THE FAMILIES AND GENERA OF BATS. 33 



remains the most important cusp, drawing the protocone and para- 

 cone toward it, so to speak, and gradually absorbing them. This 

 process may continue until the molars approximate the large premolar 

 in form, though no genus is yet known in which the protocone and 

 paracone are entirely obliterated. In the third upper molar, how- 

 ever, the paracone is the more permanent, as the successive short- 

 ening of the tooth from behind soon eliminates the metacone. In 

 all the mandibular molars the proto'conid is the chief cusp. Like 

 the metacone of m x and m z it is the first to appear as the permanent 

 teeth cut the gums, while it is the largest and most conspicuous cusp 

 throughout all changes known to occur. 



INTERRELATION OF TEETH. 



Owing to the high cusps and deep hollows which cause the crowns 

 to fit closely interlocked when the jaws are closed the interrelation 

 between the teeth of the two jaws is very intimate, so much so that 

 no modification can take place in a given region without its counter- 

 part in the opposed structures. Lateral motion of the mandible, 

 though supposed by at least one careful observer to be absent, un- 

 doubtedly occurs to the extent of permitting the posterior surface of 

 the lower canine to come in contact with the anterior surface of the 

 corresponding upper tooth. In individuals with worn teeth the front 

 face of the upper canine shows abrasion that could not otherwise be 

 accounted for. This lateral motion is sufficient to give the opposed 

 cusps and commissures of the molars the shearing action on whhh 

 their effectiveness depends. In the typical condition with the maxi- 

 mum number of teeth present the interrelation of the two sets are as 

 follows : 



Incisors. — The inner upper incisor is opposed chiefly to i ,, the 

 large median lobe of which fits into the notch formed by basal cusp 

 of upper tooth ; extreme tip of i - nearly in contact with inner edge 

 of median lobe of i ,. Outer upper incisor with long posterior sur- 

 face in contact with anterior surface of lower canine, the two teeth 

 playing against each other with a shearing motion as the jaws close. 

 The extreme tip of the tooth almost touches notch at posterior outer 

 base of i z when jaws are tightly shut. Inner lower incisor not op- 

 posed to any tooth in upper jaw. 



Canines — The chief opposition of the canines is a shearing contact 

 between the anterior inner edge of the upper tooth and the posterior 

 outer edge of lower. The point and inner surface of the upper tooth 

 also plays against the first lower premolar, or rather its inner pos- 

 terior edge cuts any food material pressed upon it by the two small 



« Gosse, Ann. and Mag. Nat. Hist., XX, p. 426, December, 1847. The animal 

 observed was a Noctilio, and the motion of the jaws is described as vertical only. 

 25733— No. 57—07 M 3 



