HOMOLOGIES OF TEETH. 375 



wards, e.g. ' one,' ' two,' ( three" ; and the premolars from behind 

 forwards, ( four,' f three,' ' two,' * one.' 



Examples of the typical diphyodont dentition are exceptions in 

 the actual creation ; but it was the rule in the earlier forms of 

 placental Mammalia, whether the teeth were modified for animal 

 or vegetable food. 



Not only the Hy&nodon, fig. 266, and Amphicyon, fig. 267, but 

 the Dichodon, Anoplotherium, Palceotherium, Cheer op otamus^ An- 

 tlirac other ium, Hyopotamus, Pliolophus, Hyracotherium, and 

 many other ancient (eocene and miocene) tertiary Mammalian 

 genera presented the forty-four teeth, in number and kind ac- 

 cording to that which is here propounded as the typical or normal 

 dentition of the placental diphyodonts. When the clue is afforded 

 to their homologies, it infallibly conducts to the true knowledge 

 of the nature both of the teeth which are retained, and of those 

 which are wanting to complete the typical number. Thus may 

 be deciphered the much modified dentition of the genus Felis ; 

 and the same clue will guide to the knowledge of the precise 

 homologies of the teeth in our own species. 



The known limits of the premaxillary in Man leads to the de- 

 termination of the incisors, which are reduced to two on each side 

 of both jaws ; the contiguous tooth shows by its shape as well as 

 position that it is the canine ; 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 in- 

 stance the secondary characters conform with the essential ones, 

 as exhibited in the dissection of the jaws of a child of about six 

 years of age, fig. 258. The two incisors on each side, d i, are 

 followed by a canine, c, 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 ad- 

 vance of it, without displacing any previous tooth, and the sub- 

 stance of the jaw contains no germ of any tooth destined to 

 displace it ; it is therefore, 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 molars, d 3 and d 4, prove them to be temporary, destined 

 to be replaced, and prove also that the teeth about to displace 

 them are premolars. According, therefore, to the rule previously 

 laid down, we count the permanent molar in place the first of its 

 series, m i, and the adjoining premolar as the last of its series, 

 and consequently the fourth of the typical dentition, p 4. 



