DEVELOPMENT OF TEETH IiN HATTERIA PUNCTATA. 193 



modifications caused by the lengthening of the incubation 

 period. 



The reason for the early appearance of the uniform teeth 

 in the mandible as compared with the maxilla and palatine 

 is undoubtedly to be found in the concrescence described in 

 Stage S; this fusion causes the whole of the mandibular 

 alternating series (not including the first three on each side) 

 to assume the character of a single multicuspid tooth, thus 

 preventing any adaptation to the growth in length of the 

 jaw. As will be seen from the comparative measurements 

 given below, the region in question does not increase in 

 length from Stage T onwards. In the adult it is represented 

 by a short apparently toothless ridge immediately posterior 

 to the large anterior tooth. When this fusion occurred 

 during the phylogeny of Hatteria, the formation of the 

 uniform teeth was hastened, and hence their early appearance 

 at the present time. I have not observed any concrescence 

 in the palatine teeth, but it probably occurs to some extent, 

 as here the uniform series begins its development much 

 earlier than in the maxilla, though later than in the mandible. 



The following is a comparative table of the number of, 

 and space occupied by, the alternating teeth or their vertical 

 successors in maxilla and mandible (three anterior teeth not 

 included) at various stages. It is usually impossible to detect 

 the separate teeth of this series in the adult mandible, so that 

 the number inserted is only an approximation. The other 

 figures are averages where possible. It will be found that 

 the number of teeth of this series is less iu the adult than in 

 the young. This is pai'tly due to the fact that successional teeth, 

 in some cases at least, replace two of their predecessors, and 

 partly to some of the teeth having become indistinguishable. 

 In the anterior mandibular region also the alternating series 

 is encroached on by the formation of the two compound 

 teeth. I have added to the table the number of uniform 

 teeth. 



Comparison of dentition of young with that of 

 adult. — We are now in a position to consider the relation 



VOL. 44, PAKT 2. — NEW SEEIES. N 



