660 BERTHA E. MARTIN 



Besides the separated darker areas described above, there are 

 always visible in the enamel a large number of fine, closely set 

 lines which run parallel to the surface of the tooth. These lines 

 evidently represent different strata of enamel deposition (fig. 

 4, FE). 



Summary. The history and fate of the enamel organ in the 

 teeth of D. novemcinctus is similar to that of other mammals. 

 A thin layer of enamel is deposited, and correlated with this fact, 

 we find unusually short ameloblasts, and an early disappearance 

 of the stellate reticulum. 



2. History of the tooth cusps 



Previous investigators of the tooth development of D. novem- 

 cinctus have said little concerning the history of the tooth cusps, 

 although they have all mentioned the fact that where two cusps 

 are present, the lingual cusp is always higher than the labial. 

 Rose states that in D. hybridus, each of the first two of the seven 

 back teeth are one-cusped, and the other back teeth are bicuspid ; 

 and in D. novemcintus all but the two anterior back teeth are 

 two-cusped. Leche states that, since in T. peba the first two 

 back teeth are one-cusped, the milk set must originally have been 

 heterodont. I have already shown that both Rose and Leche 

 mistook the last front tooth for the first functional back tooth, 

 and therefore their statement is that of the functional back teeth, 

 the first alone is one-cusped. 



a. History of the cusps in the lower jaw. In a few sections 

 through the first back tooth of my 55 mm. embryo, the dental 

 papilla appears to be slightly two-cusped. If two cusps are 

 present, the lateral one is higher, and this in addition to the fact 

 that, elsewhere in my series, there is no indication of a bicuspid 

 condition in this tooth makes it doubtful whether or not this 

 appearance is an artefact caused by unequal shrinkage in the 

 dental papilla. In all the other stages, as shown in text figure 4 

 which gives diagrammatic outlines of the teeth at birth, the first 

 back tooth is plainly one-cusped. This tooth also passes through 

 changes in shape similar to those described later for the most 



