GENERAL CHARACTERS OF THE MAMMALIA. 279 



The normal number of digits is five, but they vary from 

 one to five. The middle finger is the longest and most per- 

 sistent of the digits of the fore-limb ; and in the Horse it 

 is the only one which is left (fig. 595, a). The thumb is 

 very frequently absent. In the Kuminants there are only 

 two fingers which are functionally useful, these carrying the 

 hoofs. In many Euminants, however, there are two rudi- 

 mentary and functionally useless digits in addition. 



Normally each digit has three phalanges, except the 

 thumb, which has only two. In the Whales and Dolphins 

 {Cetacea), in which the anterior limbs form swimming-paddles 

 very like those of the IditliyosauriLs and Plesiosaurus, the 

 phalanges are considerably increased in number as they are 

 in those Eeptiles. In all the Mammalia, too, except the 

 Cetacea, it is the rule that the terminal phalanx in each digit 

 should carry a nail, claw, or hoof. 



Whilst the anterior limbs are never absent in any Mam- 

 mal, the posterior limbs are occasionally wholly wanting, as 

 in the Cetacea and Sirenia. Generally speaking, however, 

 the posterior limbs are present, and the pelvic arch has much 

 the same structure as in man. The two halves of the pelvis 

 — the ossa innominata — consist each of three pieces in the 

 embryo (viz., the ilium, ischium, and pubes), which meet to 

 form the cup-shaped cavity known as the " acetabulum," with 

 which the head of the thigh-bone articulates. In the adult 

 Mammal these three bones are anchylosed together, and the 

 two ossa innominata unite in front by means of a symphysis 

 pubis, constituted either by a cartilaginous union (synchon- 

 drosis), or by merely ligamentous attachment. In some Mam- 

 mals, however, such as the Mole, and many of the Bats, the 

 pubic bones remain disunited during life. As a rule, also, 

 the ossa innominata are firmly united with the vertebral 

 column. In the Cetaceans, in which the hind-limbs are 

 wanting, and there is no sacrum, the innominate bones are 

 rudimentary, and are not attached in any way to the spine. 



The only other bones which are ever connected with the 

 pelvis are two small bones which are directed upwards from 

 the brim of the pelvic cavity in Marsupials and Monotremes. 

 These are the so-called " marsupial bones," regarded gener- 



