n MONOPHYODONT DENTITION 53 



which arise on the outer side of the fold of epithelium aud some 

 way above its lower termination. These ultimately acquire a 

 bell-like form, and are as it were moulded on to a thickened con- 

 centration of the dermis beneath ; they then become separate 

 from the downgrowth of the epithelium whence they have arisen. 

 Finally, each of the eight germs becomes one of the milk teeth of 

 the animal. The lower end of the sheet of invaginated epi- 

 thelium, the common enamel germ, is the seat of the formation 

 of the second set of teeth, of which, however, in the animal under 

 consideration, there are only two in each jaw. But corresponding 

 to each of the enamel germs of the milk dentition, with the 

 exception of the first two molars, there is a slight thickening of 

 the end of the common enamel germ, which at a certain stage is 

 indistinguishable from the thickening which will become one of 

 the permanent teeth. We have thus the dipliyodont arrange- 

 ment. But this does not exhaust the series of rudimentary teeth, 

 though no more come to maturity than those whose development 

 has already been touched upon. In the upper jaw a small out- 

 growth of the common enamel werni arises above and to the 

 outer side of the enamel germ of the third milk incisor ; this does 

 not develop any further, but its resemblance to the commencing 

 germ of a tootli seems to indicate that it is the remnant of a 

 tooth series antecedent to the ndlk series. Furthermore, there 

 are indications in the fourth premolar of a fourth series of teeth 

 posterior in appearance to the permanent dentition. We arrive 

 therefore at the important conclusion that although here as 

 elsewhere there are only two sets of calcified teeth ever developed, 

 there are feeble though unmistakable remains of two other series, 

 one antecedent to and the other posterior to the diphyodont 

 dentition. The gap therefore which separates the mammalian 

 dentition from that of reptiles is less than has hitherto appeared. 

 Dr. Leche also carefully studied the tooth development of Iguana ; 

 he found that in this lizard there are four series of teeth which 

 come to maturity, and a rudimentary series antecedent to these 

 which never produces fully formed teeth. 



In a few mammals there is a kind of dentition known as the 

 nionophyodont, in which only one series of teeth reaches maturity ; 

 where in fact there is no replacement of a milk series by a per- 

 manent dentition. Of the nionophyodont dentition Whales form 

 an example. The ^Marsupials are very nearly an instance of tlie 



