RUMINANTS. 541 



have cut the gum, in the Lamb, in the course of the first month 

 after birth; the first true molar (m 1, fig. 4, PL 133) next appears, 

 behind the third milk molar in both jaws at the sixth month : at 

 the end of the first year the second true molar comes into place and 

 there are five molar teeth on each side of both jaws. This 

 transitional period may be detected and distinguished from an older 

 stage, when the same number of molar teeth may have been produced 

 by the loss or extraction of one of the six permanent molars, by the 

 form of the third tooth in the lower jaw (fig. 4, d3), which tooth in 

 a Sheep between one and two years old, is divided into three lobes ; 

 the permanent tooth (p 3) which succeeds it consists of one principal 

 lobe and a small posterior lobule. 



In the course of the second year the first or internal deciduous 

 incisors fall and are replaced by the broader permanent ones. The 

 state of dentition in the low^er jaw of a sheep of twenty months 

 is shown in PI. 133, fig. 4. The first permanent incisor {i 1) and 

 the first and second true molars (m 1 & 2) coexist with the 

 second and third deciduous incisors {di 2 & 3), the deciduous 

 canine (d c) and the three deciduous molars (^1, 2 & 3). 

 Towards the end of the second year, the third true molar (m 3) 

 appears above the gum, and at the same time the deciduous 

 molars begin to give place to their vertical successors, falling 

 with brief but irregular intervals, the third being sometimes pushed 

 out a few days or a few weeks before the first and second. The 

 variations in those phases of development of the molar series, 

 which are dependent on individual constitution, food, season and 

 time of birth, never prolong the process beyond the end of the 

 second year. The change of the incisors and canines is not 

 completed until a later period. The second permanent incisor 

 (z 2) displaces its deciduous predecessor in the course of the third 

 year ; the third comes into place usually during the first half oi 

 the fourth year : the outer cutting teeth of the lower jaw manifest 

 their essential nature as canines by the lateness of their development, 

 not appearing above the gum and displacing the deciduous canines 

 until the fifth year, the entire series of permanent teeth being in 

 place and use before the end of the fifth year. At the fourth 



