326 VERTEBRATE ZOOLOGY 



13. There is an external fleshy and cartilaginous conchus to the ear. 



14. The body of the vertebra is formed of three pieces, one of which 

 makes the centrum and the other two the epiphyses. 



15. Cartilaginous disks (intervertebral disks) separate the centra 

 of the vertebra from one another. 



16. The coracoid, except in the monotremes, is represented by a 

 mere vestige fused with the scapula, called the coracoid process. 



17. The ribs, except in monotremes and whales, are attached by 

 two heads, the capitulum and the tuberculum. 



18. There are characteristically seven cervical vertebrae. (In the 

 manatees there are but 6, and in the sloths there may be 6, 8 or 9.) 



~T9. With few exceptions (whales, edentates), mammals are diphyo- 

 dont, i. e., have two sets of teeth, a milk and a permanent dentition. 



20. The teeth are (a) thecodont (each imbedded in an alveolar pit 

 in the jaw bone); and (b)^heterodont (differentiated into incisors, 

 canines and molars) ; exceptionally homodont or absent. 

 ^21. There are two occipital condyles, which are part of the exoc- 

 oipital bones. 



22. Except in moriotremes there is no distinct cloaca. 

 ^23. Mammals are homothermous (warm-blooded), with a well- 

 developed temperature-regulating apparatus. 



/24. The heart is completely four chambered, with two auricles and 

 two ventricles and a complete double circulation. 



25. With the exception of monotremes, the eggs are minute in size 

 and the young are born alive (viviparous). 



There are no gills nor gill_rudiments at any stage of development. 

 -^27. An amnion and an allantois are always present; but the allan- 

 tois is sometimes vestigial or functionless. 



The first 19 characters and 20 (b) distinguish the mammals from 

 all other living vertebrates. 



Numbers 21 and 22 distinguish the mammals from modern reptiles 

 and birds (Sauropsida). 



Numbers 23 and 24 distinguish the mammals from the reptiles. 



Number 25 distinguishes the mammals from the birds. 



Numbers 26 and 27 distinguish the mammals from the Amphibia. 



The Mammalian Integument. Under this head we shall discuss 

 briefly hair; skin glands; and claws, hoofs and nails. 



The possession of hair is as truly diagnostic for mammals as are 

 feathers for birds. Even the apparently naked, glossy-skinned whales 



