CHAPTER XX. 



CLASS MAMMALIA. 



THE skeleton of the members of this class, the highest of 

 the vertebrata, has the following characteristics : 



Some part of the integument at some period of life is 

 always provided with hairs; these are epidermal structures 

 arising from short papillae of the Malpighiaii layer of the 

 epidermis, which at once grow inwards and become imbedded 

 in pits of the dermis. Sometimes scales or spines occur, and 

 epidermal exoskeletal structures in the form of hoofs, nails, 

 claws and horns are also characteristic. As regards the endo- 

 skeleton, the vertebral centra have terminal epiphyses except 

 in the Ornithodelphia and some Sirenia. In the skull the 

 cranial region is greatly developed as compared with that in 

 lower vertebrates, and whereas in many reptiles the true 

 cranium is largely concealed by a false roof, in mammals the 

 only relic of this secondary roof is found in the zygomatic 

 arch, and postorbital bar. In the adult all the bones except 

 the mandible, hyoid, and auditory ossicles are firmly united 

 together. The basisphenoid is well ossified, and there is no 

 parasphenoid. The pro-otic ossifies, and unites with the epi-otic 

 and opisthotic before they coalesce with any other bones. 



The skull articulates with the vertebral column by means 

 of two convex occipital condyles formed mainly by the ex- 

 occipitals r and the mandible articulates with the squamosal 

 without the intervention of the quadrate. The latter is much 

 reduced, and is converted into the tympanic ring, while the 

 hyomandibular of fish is represented by the auditory ossicles 1 . 

 1 This is Gadow's view ; according to Huxley the quadrate forms the 



