484 



Vertehrata. 



iP^ — jP*) J ^'^® three posterior^ molars (m^ — m^) .* In tlie complete 

 first dentition, tlie milk dentition, which, is present in the 

 young animal, but is lost after a time, there are eight teeth in 

 each half of the jaw ; three incisors {dP- — dd^), one canine {dc), 

 and four molars (cip^ — <^2'*)j occupying the places in the jaw 

 which will be filled afterwards by the corresponding permanent 

 incisors, canines, and premolars, whilst the true molars have 

 no predecessors. The number of teeth is, however, reduced in 

 many forms, and the reduction affects not only molars, but even 

 incisors and canines. Usually it is not difficult to determine, by 

 comparison, which are the missing teeth. In the molar series the 

 reduction usually begins either at the anterior or the posterior end, so 



Fig. 394. The teeth of a Pig showing the replacement, the jaws cut away, 

 t' — i^ first to third incisors, c canine, p^ — p* premolars, in} — m,^ molars, di^ second 

 deciduous incisor, dp^ — dp* deciduous molars, (milk teeth shaded). Of the deciduous 

 teeth di^, di^, dc have already fallen out ; dp^ is wanting in the Pig. — Orig. 



that if only six molars are present the absent tooth is either the 

 first premolar or the last molar; if only five are present 

 the missing teeth will be p^ and p^, or m^ and m^, or again, 

 p^ and m^. In some groups, the molars disappear first (in 

 the Seals) ; in others, the premolars {e.g., in the Rodents) ; in yet 

 others {e.g., the Cat), teeth are missing from both ends of the series. 

 The number of milk teeth may be similarly reduced ; if a tooth of the 

 permanent dentition is wanting, as a rule the corresponding milk 

 tooth has also disappeared; but there are several exceptions. Of 

 the typical deciduous molars, however, the first {dp^) is usually 

 absent, even when the corresponding permanent tooth {p^) is 

 present; occasionally other milk teeth are wanting, although the 

 permanent ones are present ; sometimes {e.g., in the Seals) they are 

 absorbed during embryonic life, or fall out at birth, and they are then 



* i' is the most anterior (the innermost) incisor, p' the most anterior premolar, to' 

 the ijiost anterior iQolar, etc, 



