294 



BACKBONED ANIMALS. 



Class VII. MAMMALIA (Milk-givers]. 



General Characteristics. We now come to the highest 

 and most perfect animal forms. They are covered with 

 hair instead of scales. The young are bom alive,* and 

 nourished by a fluid called milk, secreted in the mammary 

 glands. About twenty-one hundred species of living 

 mammals are known, three hundred and ten inhabiting 

 North America. 



Skeleton.- The skeleton, that in the majority of birds 

 is extremely light, is in the mammals solid, and the limb- 

 cavities filled with 

 marrow. Taking 

 the cat (Fig. 321) as 

 an example, we first 

 note the cranium, era, 

 or skull, that is united 

 to the backbone or 

 vertebral column by 

 two occipital con- 

 dyles. The lower jaw 

 is composed of two 



pieces and is ioined 

 ^^ ^ ^ ^ 



and not to the quad- 



rate bone, as WC have 

 ^^ J Q thg bird and 



reptiles. The back- 

 bone is divided into 



five divisions : First, the cervical or neck region, where 

 the vertebrae generally number seven. In the cat they are 

 small, in the whale they are pressed together, while in the 

 long-necked giraffe each bone is lengthened out. Sec- 

 ond, the dorsal or back region, the vertebrae of which 

 generally number from ten to fifteen ; they support the 



* See note on page 297. 



Cra 



7 89654 



FlG. 321. Cat, with bones of right side drawn. 

 Cra, cranium ; sc, scapula or shoulder- 

 blade ; i, humerus ; 2, radius and ulna ; 

 3, carpus ; 4, phalanges ; 5, femur ; 6, tibia 

 and fibula ; 7, tarsus ; 8, metatarsus ; 9, 

 phalanges ; z', innominate bone, a number 

 of bones combined, forming the pelvic 

 arch; z;, vertebral column. (After Morse.) 



