2go TOWARDS THE HUMAN FORM 



denies quot tuberculia. 1 But if in that stage of their develop- 

 ment, when they consist entirely of enamel, teeth can become 

 united by their crowns, in the later stages it is the number 

 of dental bulbs, and hence the roots that have remained free, 

 that will indicate the number of teeth united. This union 

 often occurs in an accidental manner in the simple teeth of 

 the Cetacea, and is also evident in the molars of certain 

 Marsupials, such as the Thylacine. It may happen, however, 

 that a compound tooth appears to have but one root, as in 

 the outer incisor, always marked by a notch, of the Giraffes 

 and the Okapi. This incisor results from the union, throughout 

 their extent, of two teeth, one of which is reduced almost to 

 its crown. Considerable prudence must therefore be exercised 

 in enumerating the teeth that have entered into the composition 

 of a molar, but the fact that they have been thus produced 

 by union cannot be contested, and establishes an important 

 distinction between the Theropod Reptiles and Mammals. 



The molars of living Marsupials do not all appear simul- 

 taneously. After the first dentition is established, the last 

 cheek-tooth falls out and is replaced by another behind which 

 new molars are formed. In the placental Mammals all the 

 teeth of the first dentition are replaced by others, followed 

 by the eruption of new molars. These are the molars properly 

 so-called. The cheek-teeth replaced are called pre-molars. 

 As teeth are modified with the change of diet, the mar- 

 supials can be divided, according to the form of their teeth, 

 into orders corresponding exactly to those adopted for the 

 placental animals, as follow : the Creophagi corresponding 

 to the Carnivora, the Entomophagi to the Insectivora, the 

 Rhizophagi to the Rodents, the Poephagi or Grass-eaters 

 to the Herbivora. This correspondence does not imply, 

 however, that the form and the number of the teeth correspond 

 for each group with what may be observed in placental 

 animals. In the Creophagi there are four or five pairs of 

 incisors in the upper jaw, whereas in ordinary Mammals 

 there are never more than three at the most ; hence 

 Richard Owen called the first Polyprotodontia. On the other 

 hand, the Carpophagi and the Poephagi have but a single 

 pair of incisors in the lower jaw and generally three pairs 

 in the upper, and these Owen grouped as Diprotodontia. 



1 There are as many teeth as there are tubercles. 



