TOOTH DEVELOPMENT — ^DASYPUS NOVEMCINCTUS 661 



posterior of the front teeth. During its first stages of develop- 

 ment, the entrance to the pulp cavity does not increase in size 

 at the same rate as do the other parts of the tooth. The sides of 

 the tooth bulge out because of their increase in tissue, and the 

 tooth also increases in height. As a result, the stages between 

 70 and 83 mm. give the impression of a tooth with a round dental 

 papilla, and a narrow entrance to the pulp cavity. In later 

 stages, the tooth takes on its definitive shape (text fig. 4, /) in 

 which there is a wide entrance to the pulp cavity. This change 

 in shape may mean that the ancestors of the armadillos possessed 

 teeth with a narrow opening to the pulp cavity. 



The second back tooth is at first plainly two-cusped (text fig. 

 5, 1). In the 78 mm. embryo there is but the shghtest trace of 

 the original bicuspid condition. The tooth has apparently lost 

 its two cusps by the upward growth of the groove between them 

 (text fig. 5, S). In the 82 mm. embryo (text fig. 5, 4), the second 

 back tooth possesses but one cusp, the original grove between 

 the cusps having grown upwards until it was level with the cusps. 

 In the 83 mm. embryo the groove between the cusps is plainly 

 visible (text fig, 5, 2), but as I have said before, the size of this 

 embryo is ahead of its degree of development in other respects. 

 With this one exception, the second back tooth is always one- 

 cusped after the 82 mm. stage, possessing one large rounded 

 cusp (text figs. 5, 4 and 4, g).. 



The next four back teeth (3 to 7) are always plainlytwo-cusped, 

 with a higher lingual and lower labial cusp (text fig. 4), and so is 

 the seventh back tooth with a single exception; in this case the 

 lingual cusp in both sides of the lower jaw of a 108 mm. embryo 

 has apparently so out-stripped the labial cusp in its growth that 

 the labial cusp is indicated only by a bulge on the side of the 

 tooth, and the tooth is practically one-cusped. I have looked 

 in vain for a similar condition in other embryos. In the unworn 

 teeth of an animal a few months after birth, there seems to be a 

 slight diminution of the labial cusp towards the posterior end of 

 the jaw, and in a few cases, the labial cusp of the seventh back 

 tooth is extremely low, although it is always unmistakably pres- 

 ent. It is probable, then, that this extreme diminution of the 



