334 Royal Institution. 



to the rule previously laid down, we count the permanent molar in 

 pl&ce, the first of its series, and the adjoining premolar as the last of 

 its series, and consequently the fourth of the typical dentition ; the 

 next premolar in advance being the penultimate or third of the typi- 

 cal series. 



We are thus enabled, with the same scientific certainty as that 

 whereby we recognise in the middle toe of the foot the homologue 

 of that great digit which forms the whole foot, and is encased by the 

 hoof in the Horse, to point to the second bicuspid in the upper jaw, 

 and to the first molar in the lower jaw of Man, as the homologues of 

 the great carnassial teeth of the Lion and Tiger. We also conclude 

 that the teeth which are wanting in Man to complete the typical 

 molar series, are the first and second premolars, the homologues of 

 those which were marked in the diagram of the dentition of the Bear. 

 The characteristic shortening of the maxillary bones required this 

 diminution of the number of their teeth, as well as of their size, and 

 of the canines more especially ; and the still greater curtailment of 

 the premaxillary bone is attended with a diminished number and an 

 altered position of the incisors. One sees, indeed, in the Carnivorous 

 series, that a corresponding decrease in the number of the premolars 

 is concomitant with the shortening of the jaws. Already in the 

 Mustelidee, the first premolar below is abrogated ; in Felis also 

 above, with the further loss of the second premolar in the lower jaw ; 

 the true molars being correspondingly reduced in these strictly flesh- 

 eating animals, but taken away from the back part of their series. 



If we were desirous of further testing the soundness of the foregoing 

 conclusions as to the nature of the teeth absent in the reduced dental 

 formula of Man, we ought to trace the mode in which the type is 

 progressively resumed in descending from Man through the Order 

 most nearly allied to our own. 



Through a considerable part of the Quadrumanous series, e. g. in 

 all the Old-World genera above the Lemurs, the same number and 

 kinds of teeth are present as in Man ; the first deviation being the 

 disproportionate size of the canines and the concomitant break or 

 " diastema" in the dental series for the reception of their crowns 

 when the mouth is shut. This is manifested in both the Chimpan- 

 zees and Orangs, together with a sexual difference in the proportions 

 of the canine teeth. Then comes the added premolar in the New- 

 World Monkeys, and the further additions in lower quadrupeds, xuitil 

 in the Hog genus we see the old primitive type of Diphyodont den- 

 tition resumed or retained. 



With regard to the application of the above principles and charac- 

 ters to other or newly-discovered species : — When the premolars and 

 the molars are below their typical number, the absent teeth are miss- 

 ing from the fore part of the premolar series and from the back part 

 of the molar series. The most constant teeth are the fourth premolar 

 and' the first true molar ; and, these being known by their order and 

 mode of development, the homologies of the remaining molars and 

 premolars are determined by counting the molars from before back- 

 wards, e.g. "one," "two," "three;" and the premolars /rom 6Mm«? 



