TOOTH DEVELOPMENT DASYPUS NOVEMCINCTUS 663 



tooth, the more separated are these two cusps. It is therefore 

 apparent that this tooth passes from a two- to a one-cusped 

 condition. 



b. History of cusps in the upper jaw. In the upper jaw the 

 first back tooth is always one-cusped. Judging from the appear- 

 ance of the unworn teeth of an animal after birth, the second 

 back tooth eventually becomes single-cusped by the oblitera- 

 tions of the groove between the two cusps. In the 108 mm. 

 embryo, at a time when the second back tooth in the lower jaw 

 has for some time assumed a one-cusped condition, the second 

 back tooth of the upper jaw is still plainly two-cusped, although 

 there is not as marked a distinction between these cusps as in 

 the earlier embryos, showing that the groove between the two 

 is becoming obliterated. The history of the cusps in the other 

 back teeth is similar to that in the lower jaw. 



3. Secondary tooth buds 



Prior to Tomes, Rapp, Gervais and Flower had called atten- 

 tion to the fact that among the armadillos, Tatu peba is not 

 monophyodont. Hensel, in 1872, from an examination of 

 thirty-five skulls of D. novemcinctus, found evidences of a trans- 

 ition from milk teeth to a permanent set, a change which he says 

 does not occur until the animal has nearly reached the adult 

 stage. Tomes, Kiikenthal, Rose, Ballowitz, Leche, and Spur- 

 gin, working in the embryonic development of teeth in the ar- 

 madillos, have described structures arising from the lingual side 

 of the outer enamel epithelium which they interpreted as repre- 

 senting the buds of permanent teeth — but the further histor}^ of 

 these structures has never been worked out. 



My own investigations have revealed the fact that secondary 

 tooth buds arise normally in both upper and lower jaws from the 

 Hngual side of the outer enamel epithelium. In the lower jaw 

 they are first distinguishable in a 55 mm. embryo, and in the 

 upper jaw they can be seen in a 61 mm. embryo. They appear 

 first in connection with the most anterior teeth, and last in the 

 seventh back tooth. During embryonic development, these 

 tooth buds never progress beyond the stage in which the cross 



