204 H. SPENCER HAERISON. 



possesses successional teeth belonging to the fourth dentition, 

 replacing, in one case at least, two of the alternating teeth. 

 The number of components of the large front teeth is pro- 

 bably three in each case,^ the union being produced in the 

 same way as in the premaxillary. I am uncertain how many 

 or which dentitions are represented here in the adult, as I 

 believe further change takes place in larger young indi- 

 viduals than I have examined. In the adult there is 

 posterior to the large front teeth a ridge of bone frequently 

 to all appearance edentulous. This represents the posterior 

 teeth of the alternating series, which, owing to concrescence 

 at Stage S, are unable to adapt themselves to the increasing 

 length of jaw, and lose their power of growth, becoming, 

 therefore, worn down at an early stage. There are probably 

 no teeth of the fourth or later dentitions represented here, 

 those which are formed anteriorly in the young animal taking 

 part in the compound anterior teeth. 



10. In the main dentigerous regions there is a 

 more or less prolonged cessation of tooth develop- 

 ment when the alternating series is complete. 

 Subsequently a renewed formation of teeth takes 

 place from behind forwards, and these are uniform 

 in size. In the lower jaw this halt is of short duration, and 

 the uniform teeth make their appearance long before those 

 of the palatine and maxilla. In fact, there are already five 

 uniform teeth in the mandible, when in the maxilla this 

 series is not even represented by enamel organs, and in the 

 palatines only by the first stages of two dental germs. 

 This set of teeth is no doubt a more recent acquisition, and 

 has, I believe, like the permanent molars of mammals, no 

 vertical predecessor's or successors. The want of synchron- 

 ism in its appearance in the different regions is due to 

 the fact that the concrescence of the alternating series in 

 the lower jaw prevents the teeth from adapting them- 

 selves to the growth in length, as do those of the maxilla, 

 and of the palatine (to a less extent). During the period 

 ' More probably two. See Addendum. 



