DEVRLOPMRNT OF TEETH IN HATTERIA PUNCTATA. 191 



ing. The crowding together of these two dentitions on a 

 small length of jaw, caused in the mandible that early inti- 

 mate fusion with each other which we observed at Stage S. 

 Probably, also, this process was associated with an upgrowth 

 of bony tissue round the bases of the teeth such as we find 

 at the present time in many of them. All this would render 

 the process of tooth change more expensive, and it has 

 thei'efore become much reduced, confined to the anteiior 

 region of the jaws, and restricted to the period of imma- 

 turity, when the amount of bone absorption is much less 

 than it would be in the adult. The reason for the retention 

 of an active tooth change in the anterior regions of the jaws 

 is perhaps due to the necessity for an efficient dental arma- 

 ture in the front of the moutli. 



The following diagrams will help to explain my views of 

 the relations of the tooth change in the premaxillary region 

 to the normal types of succession. Incideutnlly they suggest 

 a possible explanation for that obliquit}- of position notice- 

 able in the youngest mandibular and maxillar}^ successional 

 teeth, where these are seen to lie at the base of a small tooth 

 (second dentition), and point towards a large one (third den- 

 tition), both of which they probably replace. The explana- 

 tion suggested is, of course, that the teeth of the third 

 dentition being forced to come up between two of their pi-e- 

 decessors, gradually came to develop with their apices 

 pointing in this direction, and that this change also affected 

 the teeth of the fourth dentition. It must be understood that 

 the condition found in the first diagram is never realised, the 

 second set at the present time always alternating with the 

 first, from the earliest stages. It will be seen that, accord- 

 ing to my view, the premaxilla had primitively only one 



following table. Only tlie marginal teeth of one side are given, the pre- 

 maxillary and anterior mandibular being included. 



Upper jaw. Lower jaw 

 Stage Q (enamel organs and teeth of first dentition) 9 . 8 



Alternating teeth (second and third dentitions) . 17 — 19 . Ifi 



