366 CLASSIFICATION OF VERTEBRATES. 



crests, crescentic in character, have a longitudinal direction, the 

 tooth is selenodont. These characteristics may be combined, 

 giving the types called bunolophodont, bunoselenodont, etc. 



The teeth first formed may be the only ones to appear 

 during life, when we speak of a monophyodont condition as in 

 the monotremes, cetacea, and edentates ; or again, we may have 

 a first or milk dentition to be replaced later by a permanent set 

 (diphyodont 1 ). Milk and permanent dentitions are not exact 

 repetitions of each other, more molars appearing in the perma- 

 nent than in the milk dentition. This leads to a differentiation 

 of the molars into premolars (bicuspids of the dentist) which 

 occur in both sets, and molars proper, which appear only with 

 the permanent dentition. 



In homodont dentition the number of teeth is very large, 

 and may vary between one and two hundred. In heterodcnt 

 types this number is greatly reduced. It is greatest in the 

 marsupials, where there may be five incisors and six molars on 

 either side in either jaw. In placental mammals the incisors 

 never exceed three, and the full dentition may be stated as 

 including 44 teeth. Not infrequently, as in rodents and rumi- 

 nants, the canines may be lacking, producing a gap or diastema 

 between incisors and premolars, while not infrequently the 

 incisors may not be developed in the adult. 



To express in concise form the number of teeth present in 

 any mammal a matter of great importance in classification 

 a dental formula has been adopted, in which the kinds of teeth 

 are represented by the letters z, c, /, and ;;/, while the number 

 of teeth in upper and lower jaws are represented by figures 

 above and below a line. Since the two halves of either jaw are 

 mirror-like repetitions, only the teeth in one side are repre- 

 sented. The dental formula of the adult man is expressed 

 thus: 



i f , c |, / , m I = 32 ; 

 that of the horse, i |, c |, / }, m = 40 ; 

 that of the cow, / , c %, p 3, m } = 30. 



1 Stirling has described teeth in the marsupial Myrmecobins, formed before the milk 

 set, which, taken in connection with the studies of Kuchenthal and Rose, show that the 

 marsupials, like most other mammals, are diphyodont, and may lead to the conclusion that 

 the milk dentition must be a second set, and the permanent teeth a third. 



