SKELETON OF MAMMALIA. 65 



four cutting-edged premolars or false grinders (pm), and two flat- 

 tened true molars (m). 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 Mammalia, and for convenience of descrip- 

 tion the " dental formula " has been invented as a means of repre- 

 senting the number of each sort of tooth present in any animal. 

 That of the Dog would be I. f , C. {, Pm. f , M. f x 2 = 42, the 

 letters indicating the sort, and the numerals the number of the 

 teeth present on each side of the upper and lower jaws. 



A second dental division of the Mammalia is founded on the 

 fact that in some few forms, which are also chiefly " homodont/' 

 there is only a single set of teeth, whilst in others the adult denti- 

 tion 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 with that of the latter, the milk- 

 teeth being in some cases shed or absorbed by the time the animal 

 is born. The Dolphins, Sloths, and some Armadilloes are examples 

 of animals with only a single set of teeth during their lives ; while 

 the great majority of Mammals, like Man, have two fully-developed 

 sets, viz. the milk-dentition and the permanent, the latter suc- 

 ceeding the former in a vertical direction. 



The vertebral column consists of a variable number of ring- 

 shaped bones placed end to end, so as to form a long tube for the 

 reception of the spinal cord. The vertebrae are divided into five 

 groups (fig. 26), viz. : the cervical (cv), or those of the neck, 

 nearly invariably seven in number ; the dorsal (d) , those of the 

 back, to which the ribs are attached; the lumbar (/), or loin ver- 

 tebrae ; the sacral (s) } or those to which the hip-bones are fixed ; 

 and the caudal (cd), or those of the tail, ranging from 3 (in some 

 Bats) to 47 (in Microgale longicaudata, which is the longest-tailed 

 Mammal known). 



The ribs are curved rods of bone, from 9 to 24 pairs in number, 

 which are attached to the sides of the dorsal vertebrae, and pass 

 round the body, the greater part of them joining in front to the 

 breast-bone, or sternum, while the remainder, known as the float- 

 ing or false ribs, have their ends quite free. 



r 



