190 H. SPENOER HAKETSON. 



theory : — During the phylogeiiy of Hatteria there was pro- 

 bably a period when emergence from the egg occurred much 

 earlier than it does at present. The presence in ontogeny of a 

 complete though degenerate set of embryonic teeth, reaching 

 their maximum of development so long as nine months before 

 the end of incubation, has ah'eady suggested this to us. Let 

 us now proceed on the assumption that at one time incubation 

 terminated at this stage, and that the young animal was set 

 free with a set of these small teeth. In the ordinary course 

 these would be followed by a second set, which are now 

 represented by the smaller teeth of the alternating series, and 

 these would in their turn be displaced by the thii'd set, now 

 appearing as the larger teeth of the alternating series. After 

 these would follow, as in more normal reptiles and lower 

 types, a constant succession of teeth. For some reason the 

 period of incubation became greatly lengthened — perhaps 

 owing to difficulty in obtaining food during the winter months 

 or to changed climatic conditions^ — and the embryo, instead 

 of remaining about five months in the egg, had the period 

 prolonged to thirteen months, as at present. The embryonic 

 dentition, together with the two dentitions now represented 

 by the alternating series, were thus developed during the 

 intra-capsular period. The first set of teeth became use- 

 less, and were shed before hatching. The second set, con- 

 sisting of much larger teeth, were retained, owing probably 

 to the disadvantage of shedding or absorbing them within the 

 egg, as would be necessary if the third set were to take their 

 places. These latter, being thus unable to take their proper 

 positions, wei'e forced to occupy the intervening spaces. The 

 animal therefore left the egg with a set of teeth 

 (called in this paper the alternating series) repre- 

 senting the second and third dentitions.^ The pos- 

 terior members of both dentitions, however, suffered a 

 retardation in development, especially in the maxilla, so that 

 several of them do not now appear till some time after hatch- 



' The number of teeth of tlie first dentition, as compared with those of 

 the second and third (as seen in the alternating series), is shown in the 



