644 MAMMALIA. 



lilood vessels and nerves still enter the pulp cavity and keep the tooth 

 alive, but as the limit of growth is reached the residue of soft pulp tends 

 to disappear. Of these " rooted " teeth there are many Ivinds, differing 

 in size and shape, in the numl^er of roots, and in the period at which 

 these are definitely established. Mammals also differ not a little in 

 regard to the period at which the teeth — usually concealed at the time 

 of birth — appear on the surface or cut the gum. 



Whereas Fishes and Keptiles have a practically unlimited succession 

 of teeth, the succession in Mammals is practically limited to two sets, 

 though traces of at least a third set have been seen. It was until 

 recently the custom to distinguish between vwnophyodont Mammals, 

 with only one set of teeth, and diphyodont Mammals with two sets. 

 But more^ careful investigation has shown that there are no strictly 

 monophyodont Mammals. Even the baleen whales, which have no 

 functional teeth at all, have the rudiments of two sets. In most cases we 

 have to distinguish a more important replacing set which is functional 

 through the greater part of life, and a less important transient first set the 

 members of which, often being developed during the period of sucking, 

 are called milk teeth. The milk teeth may dwindle, as in seals, before 

 or shortly after birth ; or they may remain, as in Ungulates, for a long 

 time, being gradually replaced by the permanent set ; or they may 

 remain as the permanent dentition, in Marsupials and Cetaceans. 



Some recent investigators distinguish four generations of teeth, 

 viz. : — 



1st, or pre-milk dentition, non-functional vestiges, e.g., Myrmecobitis. 



2nd, or milk dentition, generally functional for a time, permanent in 

 Marsupials and toothed Cetaceans, usually in great part tem- 

 porary. In most Mammals, except Hyrax and a few others, 

 the first premolar is a persisting milk tooth. 



3rd, or replacing dentition, usually the permanent dentition, rudi- 

 mentary in Marsupials and Cetaceans ; 



4th dentition, doubtful, in Phoca (?), in Desmodus (?), sometimes in 

 man (?). 



By a set or generation of teeth we mean those which differentiate 

 contemporaneously, or almost contemporaneously, from the dental 

 ridge. It used to be supposed that the replacing teeth develop from 

 the milk set, but both are derived, as sister dentitions, from the dental 

 ridge. 



M. F. Woodward, in careful account of recent work, sa)-s that con- 

 sideration of the facts "leads us to the belief (i) that the livino- 

 Mammalia show traces of from three to four distinct generations of 

 teeth, and consequently (2) that they are potentially polyphyodont ; 

 (3) that the first set is vestigial and not functional in any Uving 

 mammal ; (4) that the second, which is so important in the lower 

 mammals, is more or less replaced by the third in the higher forms ; 

 and (5) that this third dentition remains dormant in the Ma'rsupials and' 

 Cetaceans." 



Mr. Woodward points out that the evolution of the specialised 

 heterodont dentition of the Mammalia from the simple homodont and 

 polyphyodont dentition of the lower Reptilia " would necessarily cause 

 a reduction in the number of successional sets of teeth, due to an 



