Royal Institution. 333 



and the complex structure of the true molars, such premolars are 

 distinguished hy the same character of development as those of the 

 Artiodactyla, or Ungulates with toes in even number ; although 

 here the premolars are distinguished also hy modifications of size 

 and shape. 



In most of the South American Quadrumana, the number of teeth, 

 as contrasted with the Monkeys of Africa and Asia, is increased to 

 thirty-six, by an addition of one tooth to the molar series on each 

 side of both jaws. It might be concluded a priori ^ that as three is 

 the typical number of true molars in the placental Mammalia with 

 two sets of teeth, the additional tooth in the New- World Monkeys 

 would be a premolar, and form one step to the resumption of the 

 normal number (four) of that kind of teeth. The proof of the accu- 

 racy of this inference was given by the state of the dentition in the 

 young of the Howler-Monkey (Mycetes), in which a diagram was 

 exhibited of a dissection of the jaws, exposing the germs of the per- 

 manent teeth : the crown of a premolar being found above the 

 third milk-molar in place, as well as above the second and ^rst. As 

 regards number, therefore, the molar series, in Mycetes, is interme- 

 diate between that of the Bear, Ursus, and Felis ; the little premolar 

 p. i. in Ursus, tells plainly enough which of the four is wanting to 

 complete the typical number in the South American Monkey, and 

 which is the additional premolar distinguishing its dental formula 

 from that of the Old- World Monkeys and Man. 



With regard to the Human Dentition, the discovery, by the great 

 poet Goethe, of the hmits of the premaxillary bone in man, leads to 

 the determination of the incisors, which are reduced, as in Apes and 

 Monkeys, to two on each side of both jaws ; the contiguous tooth 

 shows by its shape, as well as position, that it is the caniHC ; and the 

 characters of size and shape have also served to divide the remaining 

 five teeth in each lateral series into two bicuspids and three molars. 

 In this instance, as in the dentition of the Bear, the secondary cha- 

 racters conform with the essential ones. But since we have seen of 

 how little value shape or size are, in the order Carnivora, in the 

 determination of the exact homologies of the teeth, it is satisfactory 

 to know that the more constant and important character of develop- 

 ment gives the requisite certitude as to the nature of the so-called 

 bicuspids in the Human subject. The condition of the teeth was 

 shown in the jaws of a child of about six years of age. The two incisors 

 on each side are followed by a canine, and this by three teeth having 

 crowns resembling those of the three molar teeth of the adult. In 

 fact, the last of the three is the first of the permanent molars; it 

 has pushed through the gum, like the two molars which are in advance 

 of it, without displacing any previous tooth, and the substance of the 

 jaw contains no germ of any tooth destined to displace it ; it is there- 

 fore, by this character of its development, a true molar, and the germs 

 of the permanent teeth, which are exposed in the substance of the 

 jaw between the diverging fangs of the two anterior molars, prove 

 them to be temporary, destined to be replaced, and prove also that 

 the teeth about to displace them are premolars. According, therefore. 



