MAMMALS. 365 



absence of a pelvis in these forms allows no line to be 

 drawn between lumbar, sacral, and caudal regions. 



In the skull there is a tendency for bones which are 

 distinct in the fishes and reptiles to fuse with each other, 

 so that the number of distinct elements is considerably 

 reduced. The skull is borne on the first cervical vertebra 

 (atlas) upon which it slides by means of two rounded sur- 

 faces or condyles. The lower jaw articulates directly with 

 the skull, and is never suspended by a quadrate bone, as 

 in all other classes of vertebrates, the cyclostomes excepted. 



FIQ. 157. Brain of dog. (After Wiedersheim.) II-XII, the cranial nerves 



(see page 299). 



The fore limbs are always present; the hind limbs are 

 absent in the whales and manatees, being represented in a 

 few forms by one or two bones imbedded in the muscles of 

 the trunk. Except in the Monotremes (p. 369), the cora- 

 coid does not occur as a distinct bone, but as a small 

 prominence joined to the shoulder-blude (scapula), while 

 in many the collar-bone (clavicle} also is lacking. The feet 

 have typically five toes, but not infrequently this number 

 is reduced by a disappearance of the outer digits, the 

 reduction reaching its extreme in the cow, which has but 

 two, and the horse, which walks upon the tip of its middle 

 toe. 



