TEETH. 



933 



molar series in the Quadrumana and Man, we 

 find that a tooth which displaces and succeeds 

 a milk-tooth in one species is made the homo- 

 logue of a tooth, which, in Man and Quadru- 

 mana, rises above the gum without displacing 

 any predecessor : in other words, the " prin- 

 cipale " is a premolar in certain genera, and a 

 true molar in other genera. I may refer to 

 my Paper on the Classification of the Molar 

 Teeth in the " Annales des Sciences,"* and to 

 the concluding pages of the chapter on the 

 teeth of the Carnivorain my " Odontography " 

 (p. 514), in proof that a " molaire principale " 

 does not exist in nature ; that the characters 

 by which it is defined by M. de Blainville are 

 artificial ; and that they fail in their application 

 to determine the teeth in the series of pla- 

 cental Mammalia with deciduous and perma- 

 nent teeth. 



In the series of figures, fig. 580., the 

 continuous line traverses the tooth or its 

 homologue, in Man and the Ruminant, which 

 Cuvier distinguished as the " molaire carnas- 

 siere:" the dotted line traverses that tooth 

 which M. de Blainville distinguishes as the 

 " molaire principale : " the letters and numbers 

 symbolise the teeth and indicate their indi- 

 vidual homologies, and the binary division of 

 the molar scries, which it has been one object 

 of the present Article to illustrate. I shall 

 conclude it by showing how these symbols 

 may be applied to the exposition of facts in 

 the comparative anatomy of the teeth, and for 

 that purpose select the complex and intricate 

 subject of the succession of the teeth in the 

 kangaroo. 



The chief modifications of the marsupial 

 dentition are described and illustrated in the 

 article MARSUPIALIA (Vol. III. pp. 258 298.). 

 When that volume was published I had not 

 had the means of tracing the period and order 

 of the development and succession of the 

 entire series of teeth in any of the marsupial 

 genera. The first of the five grinding teeth 

 on each side of the jaws of the wombat had 

 shown, by its displacing a milk-tooth vertically, 

 that it was a premolar ; and the adjoining 

 molar, by its earlier development and use, was 

 plainly the first of the four true molar teeth. 

 In the carnivorous and insectivorous families, 

 the marked difference of form and size of the 

 last four teeth from those intervening between 

 them and the canines, had also induced me to 

 class them as true molars, although I had not 

 got the developmental evidence of the fact, 

 except in the case of the Kangaroos and 

 Potoroos (Macropus and Hypsiprymnus). 

 The analogy, however, seemed to be sufficient 

 to justify the generalisation that the Marsupial 

 differed from the Placental Diphyodont mam- 



mals in having four true molars, L e., m. 



33 



44- 



instead of m. ^- and also that they differed 



in having only three premolars, i. e, p. ; 



Tom. iii. (1815), p. 116. 



33 



33 



44 



instead of p. ^-^ ; the typical number of the 



7 _ 7 

 grinding series, ~^., being the same. The 



genus Myrmccobius offered the most remark- 

 able exception here, as the Manatee had done 

 in the placental series, in the increased number 



c , 88 99 



of the grinding teeth, e. g. to g ^ or -, 



which, according to the shape of the crowns, 

 were divided, in the Myrmccobius^ into p. 



O O f f* 



jjHy m - gl 6 ' ^ Ut l ^ e ort ' er f development 

 and succession may show that the number of 

 premolars is greater, and that of the true 

 molars less. The probably marsupial Tliyla- 

 cotherium or Amphithcnum from the Oxford 

 oolites the most ancient of all known mam- 

 mals had as many as twelve teeth in each 

 molar series, besides a canine and three 

 incisors, and by their form I have grouped 

 33 1 1 66 66 



t . . 



them as : , , ,. 



, p . . _ 



, 



An interesting field of observation still re- 

 mains open in regard to the period and order of 

 developement of the deciduous and permanent 

 teeth, in the different carnivorous, omnivorous, 

 insectivorous, and frugivorous Marsupials. At 

 present I have ascertained the required facts 

 only in the herbivorous family (Poephaga). 



The permanent dental formula of both the 



g _ 3 



Macropodida; and Hypsiprymnidae is, /'. - -- -, 



1 1 11 44 

 c - ' P- m ' = 30 - Thc 



which are confined to the upper jaw, are 

 small or minute when retained ; and disappear 

 after being represented " en germe" in most 

 of the true kangaroos. 



The deciduous dentition of the great Kan- 



g _ o i _ i 



garoo (^Macropus major) is, i. - > c. 



- 





22 



= 18. The canines are rudimental, 



and are absorbed rather than shed. The 

 deciduous incisors are shed before the young 

 animal finally quits the pouch : when this 

 takes place, the dentition is : 



, A m 1 9 



i_i' a ' l '2-2~ 



the upper incisors being i. 1, the molars d. 3 

 and d. 4 of the typical dentition. This stage 

 is exemplified in the lower jaw at A, fig. 594. 

 The next stage shows the acquisition of i. 2 

 in the upper jaw, and m. 1 in both jaws, and 

 the formula is : 



At one year old, the dentition is : 



33 . 22 22 



i. . rt.w. , m. = 24 ; 



1 I 22' 22 



the additional teeth being i. 3 and m. 2 (c, 

 fig. 594. ; in which the demonstration of the 

 true deciduous character of d. 4 and d. 3 is 

 shown by the germ of their vertical successor, 



' Odontography, p. 376, \<L 09. 

 3 o 3 



