TOOTH DEVELOPMENT — DASYPUS NOVEMCINCTUS 649 



sectioning, and by the time decalcification was complete, they 

 were of little use for histological purposes. I was able, however, 

 to obtain a few good sections which have proved of great value. 

 This gap in the series between birth and late post-embryonic 

 stages is most regretable but has not seriously interfered with 

 the understanding of the tooth ontogeny. As will be seen, the 

 condition at birth and the stages leading up to this condition 

 give a clear prophecy of coming events so that with the facts 

 which may be gleaned from the post-embryonic stages one may 

 be sure that he has a very accurate picture of what has happened 

 between the two stages. 



B. DESCRIPTION OF TEETH IN POST-EMBRYONIC SKULLS 



Before proceeding to the review of the literature or to the 

 results of my own investigations, it may be well to describe 

 briefly the facts concerning the dentition of Dasypus novemcinc- 

 tus as they may be ascertained from a survey of the dried skulls 

 (text fig. 1). The teeth occur in the posterior part of the skulls 

 and the dental formula is 7/7 or 8/8, depending on whether or 

 not the last back tooth has been erupted, as this tooth erupts 

 some time after the others. As this last tooth erupts in the lower 

 jaw before it does in the upper, at certain stages the dental for- 

 mula may be 7/8. The teeth are so arranged in the jaws that 

 the first front tooth of the lower jaw has no corresponding tooth 

 in the upper jaw with which to articulate (see text fig. 1) ; the 

 second front tooth of the lower jaw articulates with the first 

 front tooth of the upper jaw. The teeth alternate with each 

 other as shown in text figure 1, so that the last back tooth of the 

 lower jaw articulates with the next to the last back tooth of the 

 upper. 



Although it is maintained that in Dasypus peba the first upper 

 front tooth is situated in the premaxilla, I can state with cer- 

 tainty that both in the embryonic and post-natal stages of D. 

 novemcinctus the first upper front tooth is situated in the maxilla, 

 and some distance back from the premaxilla so that a consid- 

 erable diastema is left between the two. This appears plainly 

 in text figure 1. 



