238 MAMMALIAN DENTITION 



and all are obviously cheek teeth. Fig. 84 reveals the fact that 

 although little can be made out concerning the first tooth the 

 second presents in masked form the general characters of the 

 tritubercular tuberculo-sectorial type. (The third is not shown.) 

 The upper tooth possesses a trigon the large protocone of 

 which is connected by ridges with paracone and metacone and 

 in addition there is a well-developed liypocone. The outer 

 border of this tooth like the inner margin of the corresponding 

 lower molar is complicated by numerous small secondary tuber- 

 cles. The second low T er molar presents a paraconid and an ele- 

 vated protoconid though the metaconid and the entoconid are 

 obscure. The heel on the other hand, shows a large hypoconid. 

 The teeth are much broader than high and their vestigial roots 

 penetrate the underlying growing horny layer to reach the 

 bone in which however they are not deeply imbedded. 



Misled by the tuberculate appearance of the teeth, some 

 authors have likened them to those of the Triassic Multituber- 

 culate Marsupial Microlestes. The similarity in appearance 

 between these two is merely superficial. The tooth structure in 

 Ornithorhynchus is profoundly different from that in Micro- 

 lestes. On the ground that the Monotremes represent the low- 

 est phase of mammalian life other writers have sought to find 

 in the teeth of Ornithorhynchus indications of what must have 

 been the dentition of the early Mammals. This is plainly a false 

 trail for an animal so aberrant and specialized with an. ances- 

 tral history reaching back to the very dawn of mammalian life, 

 and especially of such peculiar feeding habits as Ornithorhyn- 

 chus could not be expected to retain teeth at all approximating 

 in appearance the typical ancestral mammalian form. There 

 is then no great difficulty in accepting for the teeth of this 

 animal a tritubercular tuberculo-sectorial origin. 



