664 BERTHA E. MARTIN 



section presents the appearance of a narrow column of cells 

 (figs. 7 and 9) possessing an outer layer of columnar cells and an 

 inner layer of long narrow cells with their long axes at right 

 angles to those of the cells of the outer layer. This lack of 

 activity on the part of these tooth buds during embryonic life is 

 associated with the well known fact that the permanent teeth 

 are not erupted until relatively late in the life of the animal. 



The gap in my series between birth and a stage several months 

 after birth leaves us ignorant as to the intermediate stages in the 

 development of these permanent teeth, but sections through the 

 permanent teeth of the animals a few months after birth (fig. 10) 

 show that their development must have been similar to that of 

 their milk predecessors. They are rootless, of persistent growth 

 and possess normal tubular dentine. These permanent teeth 

 also possess a thin layer of enamel. Enamel deposition is still 

 in progress, so that I do not know how much is formed. 



In the young animals the permanent teeth are found lying in 

 grooves on the lingual side of the first seven back teeth. These 

 grooves have been formed by the absorptive action of the per- 

 manent teeth. Text figure 7 shows the relation of the perma- 

 nent and milk teeth. Tomes shows a picture of a still later stage 

 in which the permanent tooth has absorbed the entire center of 

 the base of the milk tooth. In the Field Museum of Natural 

 History, I have examined a large number of skulls in which the 

 permanent teeth were erupted. In some of these teeth, I found, 

 at both anterior and posterior ends, a thin scalelike remnant of 

 the milk tooth. This proves that the permanent tooth grows 

 up through the center of its milk predecessor. The most an- 

 terior of the permanent teeth is one-cusped; the others are bi- 

 cuspid with a higher lingual and lower labial cusp (text fig. 8) . 



E. DEVELOPMENT OF THE NON-FUNCTIONAL FRONT TEETH OF 



THE LOWER JAW 



From a review of the literature it can be seen that almost 

 everyone who has studied tooth development in the armadillo 

 has described a varying number of poorly developed tooth germs 

 in the anterior part of the lower jaw. Spurgin is the only inves- 



