202 H. SPENCKR HARRISON. 



form the labio-dental grooves, that of the palatine an external 

 groove bounded labially by a prominent ridge (crista medi- 

 alis) between maxilla and palatine. 



4. The teeth which are developed on the dental 

 lamina during the incubation period, and which 

 function during the early life of the young animal, 

 are almost certainly the members of two distinct 

 dentitions (the second and third), the later teeth 

 instead of displacing the earlier coming to alter- 

 nate with them. The chief arguments in favour of this 

 view are — (a) In the upper jaw, at Stage R, every alternate 

 tooth is small, is further advanced in development, and has 

 a lingual prolongation of the dental lamina ; these belong to 

 the second dentition. The other teeth, on the contrary, have 

 larger enamel organs, are in an earlier stage of development, 

 and have no lingual prolongation of the dental lamina (third 

 dentition). In the lower jaw it is chiefly the alternation in 

 size which indicates the two dentitions, (b) The number of 

 teeth in the alternating series is eventually approximately 

 double that of the first set. (c) The first successional teeth 

 of the young animal, in several cases, each replace a large 

 and a small tooth of the alternating series. That is, a member 

 of the fourth dentition displaces not only a member of the 

 third, but also one of the second. On the other hand, as 

 seen in the premaxiUa, a member of the fifth has only one 

 predecessor of the fourth. 



5. Successional teeth make their first appearance in the 

 prem axillae some months after hatching. In this region 

 there are in all five distinct sets of teeth repre- 

 sented during development to a length of 21*2 cm., 

 commencing with the first or embryonic series. Each pre- 

 maxiUa originally bore one tooth only, with a constant suc- 

 cession ; the presence of three in the newly-hatched animal 

 is due to the second and third dentitions functioning at the 

 same period, the premaxilla then having also a tooth of the 

 third dentition belonging to the maxilla. The number is 

 reduced to two by the displacement of the premaxillary teeth 



