SKELETON OF MAMMALS. 
9 
axis o£ the skull ; but in some orders_, such as the liodentia^ 
the condyle is lengthened from front to back, and works in a 
corresponding longitudinal depression in the base of the cranium. 
The tongue-bones, or hyoid apparatus (A), consist of a series 
of small bones suspended from the posterior part of the 
cranium, and supporting the larynx and root of the tongue. 
The dentition of Mammals is of two kinds. In some few 
groups all the teeth are of one type or pattern, as in the Sloths, 
Armadillos, Dolphins, &c. ; but the great majority are pro- 
vided with teeth of several different types. Thus in the Dog’s 
skull (fig. 2) the three small teeth fixed on each side in the 
premaxilla (ynna’) are the incisors, or cutting-teeth (/) ; next 
follows a long and powerful tooth known as the canine (c). 
Behind this there are four cutting-edged premolars {pm) and 
two fiattened true molars (»?). In the lower jaw the same 
types of teeth are represented, there being in the Dog three 
incisors, one canine, four premolars, and three molars. These 
numbers vary greatly in the different orders of Mammals, and 
for convenience of description the “ dental formula ” has been 
invented as a means of representing the number of each sort of 
tooth present in any animal. That of the Dog would be — 
I* i? P* 4 > M. f X 2 = 42, the letters indicating the 
sort, and the numerals the number of the teeth present on 
one side of the upper and lower jaws. 
A second dental division of the Mammals is founded on the 
fact that in a few groups there is only a single set of teeth, 
Avhilst in others the adult dentition is preceded by an earlier set, 
named the ‘^milk ” dentition, on account of its generally being 
present during the period in which the young animal is 
nourished by the milk of its mother, although its duration does, 
not coincide Avith that of the latter, the milk-teeth being in 
some cases shed or absorbed by the time the animal is born. 
Dolphins, Sloths, and some Armadillos are examples of 
animals with only a single set of teeth during their lives ; but 
the great majority of Mammals, like Man, have tAvo fully- 
developed sets, viz. the milk and the permanent dentition, the 
latter succeeding the former in a A^ertical direction. 
The backbone, or A^ertebral column, consists of a A^ariable 
