206 H. SPENCER HARRISON. 



wliicli is shed before hatching, as well as of the fact that the 

 succeeding set is approximately double the number of the 

 first, and therefore (as is strongly supported by many other 

 arguments) represents two dentitions, which in consequence 

 of their development within the egg have come to form an 

 alternating series. The crowding together of these on too 

 short a length of jaw explains the concrescence which takes 

 place in the mandible : this, again, is the cause of the early 

 appearance of the uniform teeth in this region and their 

 greater number in the adult. It is possible also that the 

 origin in phylogeny of the uniform series, and the other 

 irregularities of tooth succession, may be traced to this 

 lengthening of the incubation period. 



Addendum. 



Some time after the foregoing paper was sent to press, I 

 was enabled, through the kindness of ray chief, Professor 

 W. N. Parker, to examine the prepared skull of a young 

 Hatteria, whose total length was 24*6 cm. The skull 

 measures 3*5 cm. from premaxilla to occipital condyle. The 

 dentition forms an interesting intermediate stage between 

 the last described and that of the young adult, and a study 

 of it has caused me to slightly modify some of my previous 

 opinions. 



The character and disposition of the teeth is shown in the 

 following table, one side only being given. The smaller 

 teeth of the alternating series are so worn, even at this 

 stage, as to be difficult of identification. 



Successional. Alternating. Uniform. 



Premaxilla . 2 ... — ... — 



Maxilla . . 3 ... 10—12 ... — 



Palatine . . 1 ... 4—5 ... 2 



Mandibular . 2 ... 7 ... 8 



In the premaxilla the outer tooth has obviously only 



