MAMMALIA 325 



status of the human mammal. Without Man the mammals would 

 have at best a disputed claim to highest rank among the vertebrates; 

 with Man included, the mammals rejgn supreme. 



The mammals are to-day as well defined in their structural char- 

 acters as are the birds, although many of them are exceedingly aber- 

 rant. No transitional type at all on a par with Archceopteryx exists, 

 for the mammals, although the monotremes are intermediate in some 

 particulars between the typical mammals and the reptiles, and the 

 reptilian order of cynodonts shows many mammahan traits. 



Distinguishing Characters of Mammals 



1. The skin (Fig. 167) is more or less clothed with hair, though in 

 some species there are only a few localized bristles. 



2. A muscular diaphragm forms a complete partition between the 

 thoracic and abdominal parts of the body cavity; it functions both 

 in respiration and in parturition. 



3. Mammary glands, with or without teats, are always present. 



4. Sebaceous glands and sweat glands are always present (Fig. 167). 



5. The red blood corpuscles are non-nucleate in the definitive con- 

 dition and are circular in outline, except in the camels where they are 

 ovoid. 



6. The cerebral hemispheres are connected by a heavy commissure 

 of fibers, called the corpus callosum (Fig. 171) which is rudimentary 

 in monotremes and small in marsupials. 



7. There is a single aortic arch, the left, which curves over the left 

 bronchus. 



8. A larynx, or voice-box, lies at the upper end of the trachea. 



9. An epiglottis, a movable cartilaginous plate, covers the glottis. 



10. Lips and cheeks are characteristic of all mammals except 

 whales. 



11. The mandible (Fig. 169) consists of but one pair of bones, the 

 dentaries, which are firmly fused in the adult; the dentary articulates 

 directly with the squamosal. 



12. There is a chain of three bonelets in the middle ear that con- 

 nects the tympanum with the inner ear. These bonelets are: the 

 stapes (believed to be homologous with the columella of reptiles), 

 the malleus (believed to be homologous with the articulare of rep- 

 tiles), and the incus (believed to be homologous with the quadrate of 

 reptiles). 



