GENERAL CHARACTERS OF THE MAMMALIA. 4OI 



mentary metacarpals ; but by Cuvier they were looked upon as 

 imperfect fingers. In most of the other Ungulates there are at 

 least three metacarpals, and in the Elephants there are five. 



The normal number of digits 

 is five, but they vary from one 

 to five. The middle finger is 

 the longest and most persistent 

 of the digits of the fore-limb ; 

 and in the Horse it is the only 

 one which is left (fig. 334, A). 

 The thumb is very frequently 

 absent. In the Ruminants there 

 are only two fingers which are 

 functionally useful, these carry- 

 ing the hoofs. In many Rumi- 

 nants, however, there are two 

 rudimentary 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 Ichthyosaurus and 

 Plesiosaurus, the phalanges are 

 considerably increased in num- 

 ber as they are in those Rep- 

 tiles. 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 Mammal, 

 the posterior limbs are occasionally wholly wanting, as in the 

 Cetacea and Sirenia. Generally speaking, however, the poste- 

 rior 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 inno- 

 minata unite in front by means of a symphysis pubis, con- 

 stituted either by a cartilaginous union (synchondrosis), or by 

 2 c 



A B 



Fig. 334. A, Fore-leg of the Horse : 



Radi 



Splint-bon 



pastern 



c Carpus ; ca Canon-bon 

 e ; a First phalanx or " great 

 b Second phalanx or " small 

 pastern ; " c Ungual phalanx or " coffin- 

 bone." B, Fore-limb of a Deer : r Radi- 

 us ; c Carpus ; ca Canon-bone ; s Supple- 

 mentary toe. 



