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NORTH AMERICAN FAUNA 74 



In connection with my investigation of methods for age determination, I 

 examined also the ontogeny and succession of the teeth, as well as their 

 morphology, structure, growth, and function. Many of my findings contribute to 

 further understanding of those aspects and relate especially to current questions 

 of phylogeny, anatomy, development, and feeding. 



Definitions 



The principal references consulted in connection with these studies were 

 Kronfeld (1937), Schour (1953), and Scott and Symons (1961). The terminology 

 used here is as follows: 



• A primanj tooth is a tooth of the first or "milk" dentition; it has no 

 antecedent. Primary teeth are indicated by lower case letters: i, c, p, and m. 



• A secondary tooth is a tooth of the second or "successional" dentition; it is a 

 tooth that has a primary antecedent. A secondary tooth usually develops on the 

 lingual side of its primary antecedent and eventually replaces it. Secondary teeth 

 are indicated by capital letters: I, C, and P. There are no calcified secondary 

 molars. 



The teeth are designated as incisors, canines, premolars, and molars. Starting 

 from the anterior midline of the upper and lower jaws, they are identified as 

 follows: 



•The incisors (i, I) are the only teeth in the upper tooth row that are rooted in 

 the premaxillae; in the lower tooth row they are the only teeth anterior to the 

 canines, from which they are usually separated by a diastema slightly greater 

 than the spaces between the incisors themselves. Except for P, all of the incisors 

 of the walrus are very small, vestigial teeth and usually are resorbed early in life. 



• The canines (c, C) are the largest and most deeply rooted teeth. The upper 

 canines, situated anteriormost in the maxillae and lateral to the main tooth row, 

 are called tusks because of their large size. The lower canines are aligned with 

 the premolars and usually are slightly longer rooted but of about the same 

 diameter. 



• The premolars (p, P) are the postcanine teeth that antecede (primary) or are 

 replaced by another tooth (secondary) during dental succession. 



• The molars (m) are small, vestigial teeth, situated posterior to the premolars 

 and often separated from them by a slightly wider space than that between the 

 premolars themselves. These are the last teeth to develop in the primary denti- 

 tion, and they are the only primary teeth that are never replaced by secondary 

 successors. They usually are recognizable by the fact that their roots are slightly 

 offset laterally from the secondary tooth row. 



The surfaces of a tooth are: lingual toward the tongue, labial toward the 

 cheeks and lips, occlusal toward the opposing teeth, and apical about the root 

 apex. 



A tooth is said to have a crown and root. The anatomical crown is the part 

 with an outer covering of enamel. The anatomical root is the rest of the tooth, 

 proximal to the anatomical crown and having an outer covering of cementum. 



