186 



H. SPENCER HARRISON. 



this stage. lu sections of the right maxilla I have found 

 that at the base of the first successioual tooth a successor 

 has commenced to form. The indications of an active tooth 

 change in this portion of the maxilla are therefore very- 

 marked. The third successional mentioned here is no doubt 

 the one discovered by Baur (7) in the specimen in which he 

 found also the first successioual tooth in the premaxilla, and 

 the first in the mandible (to be presently described). He 

 appears, however, to have worked from dissection only, and 

 it is difficult to see how he arrived at the conclusion that 

 these teeth never become functional. The "alveolus" he 

 alludes to as containing the teeth was no doubt the cavity 

 caused by bone absorption. His specimen was evidently 

 slightly younger than the one under consideration. 



From the foregoing it is clear that there are at least four 

 successional teeth developed in the anterior part of the 

 maxilla. Sections through the posterior maxillary tooth 

 show that this is large, well developed, and completely fused 

 with the bone. The dental lamina is continued beyond this 

 point for a short distance, but has no traces of enamel organs. 

 From this fact, and other considerations to be presently 

 adduced, I conclude that there is a halt in the formation of 

 maxillary teeth, commencing when about sixteen ^ of the 

 alternating series have been produced, and lasting for a con- 

 siderable period beyond the stage of the specimen now 

 described. It follows that these teeth (all of the alternating 

 series) or their vertical successors, when present,are functional 

 during a large part of the period of growth of the animal. 

 Subsequently new teeth are added from behind forwards, 

 and these are uniform in size. At this stage they have not 

 commenced their development. 



Each palatine has seven teeth, which show differences in 

 size but no very regular alternation. In a series of sections 

 through this region I find the enamel organ of a succes- 

 sioual tooth situated liugually to the second functional, the 

 usual absorption of bone being in progress. The dental 

 ' Probably more, 18 — 19. See Addendum. 



