"36 Bulletin American Museum of Natural History [Vol. XLVII 



is directed forward, as is the case with the incisors, which it exceeds but 

 little in size, its blunt-convex crown not rising above the crown surface 

 of the incisors. P] is a 2-rooted unicuspid, perfectly caniniform in struc- 

 ture and function; its height is about twice its basal length, or about 

 twice the height of p 2 and p 3 , and it is a persistent tooth of the first set. 

 P2, P3, and p4 are respectively similar in structure to their predecessors 

 but more massive. P 4 conforms in plan with the molars except in having 

 an additional cusp on the interior border, making five cusps instead of 

 four as in the molars. It is thus a slightly longer tooth than mi. The 

 median internal cusp, when the tooth is unworn, is usually minutely 

 bipointed, as in its predecessor. Mi and m 2 are similar in structure, but 

 m 2 is only about half the size of mi. 



Development and Replacement of the Deciduous Teeth 

 The condition of the teeth, as seen in the cleaned skull at the stage 

 when the crowns of the last three premolars in each jaw ( ^2, dps', dp 4 ) 

 are fully excluded and the teeth have become functionally effective (skull 

 No. 49513), is as follows. The incisors and canines (both above and 

 below) are, like the last three premolars, fully developed and functional; 

 the extreme tip of p 1 is barely above the alveolar border and would be, 

 in most cases at least, still covered by the gum; dp 2 is not visible but 

 there is a narrow slit at the outer base of the convex diastema between 

 the canine and dp 2 ; there is also a narrow opening in the alveoli of the 

 future molars, in each jaw. 



Upper Teeth. — Later (Fig. 1, Stage 4), after the skull has nearby 

 doubled its size, p 1 attains its full development. P 1 persists without 

 change except by wear, and, later in life, is the first tooth to become in- 

 efficient through excessive attrition. After considerable further increase 

 in the size of the skull the crown of the first molar is excluded and, as 

 soon as it has reached functional maturity, is followed by the second 

 molar. Not, however, till m 2 has become fully functional is there any 

 further change, when the crowns 1 of the permanent canine and the 

 permanent premolars (p 2 , p 3 , p 4 ) can be seen between the roots of their 

 predecessors, but it is considerably later before the milk premolars are 

 finally shed and their successors are fully developed. 



Lower Teeth. — The first change in the milk teeth of the mandible 

 occurs coincidently with the breaking of the crown of mi through its 

 alveolus and before the crown surface of the tooth is much above the 



l ln skull No. 49428 the crown of the right canine is double, consisting of two equal slender stiles in 

 close contact » 



