as indicative of the Age of the Animal. 343 
Fiji. 41.' 
stances at a year and three quarters old, the second pair of per- 
manent incisors 2, 2, being up, but not as yet fully developed. 
We now come again to the molars. 
Our last illustration of these teeth (fig. 
40) showed the sixth molar cuttiiif/ at 
eighteen months. This tooth attains 
its proper height by the time the sh»ep 
is two years old. After its cutting the 
three temporary molars are replaced 
by the permanent nearly all together, 
and not following any definite order in 
giving place to their successors. Fre- 
quently the second falls before the first, 
but the third is rarely changed before 
either of the other two. In figure 42 
I have exhibited one-half of the lower 
jaw of a sheep at two years of age. 
Here tlie sixth molar has gained the level of the others, and the 
two anterior temporary ones are replaced by the permanent. The 
third temporary molar is in the act of giving place to its suc- 
cessor. The central part of its body has been absorbed by the one 
beneath it, but upon this tooth its crown still adheres, confined 
in that situation chiefly by the hold it still has with the gum. 
Subsequent to these changes being perfected, usually by two years 
and a quarter, the molar teeth can only assist our judgment by the 
amount of wear they have undergone, as shown by the comparative 
flatness of their surface, and the width between the outer and 
inner reflections of the enamel. 
From two years we pass to tivo and a quarter, when the earliest 
cases of six permanent incisors are found. The third pair of per- 
manent teeth, does not, even in sheep of car/i/ dentition, succeed 
the second in the same time as those teeth did the first ; there 
being six months only between the cutting of the first and the 
second pair, but nine between the second and third. It is some- 
what difficult to account for this difference. It may be that the 
small size of the jaw of the animal, and great amount of room 
required by the perfecting so quickly of the first and second 
pairs, together with the concentration of the developing process 
in them, retard the growth- of the third pair, and that thus the 
break in the regularity is produced. In sheep of late dentition, 
as has been explained, the second pair of permanent incisors is 
not in the mouth till two years ; and in these same animals the 
third pair is sometimes not cut till about two years and three 
* Fig. 41. Front part of the lower jaw of a sheep one year and three quarters 
old. showiug the second pair of permanent incisors, 2, 2, in situ. Natural size. 
