or THE CLASS MAMMALIA. 8 



and the alveolar processes. The jaws of Mammals with few ex- 

 ceptions are provided with teeth, which are arranged in a single 

 row; they are always lodged in sockets, and never anchylosed 

 with the substance of the jaw. The tongue is fleshy, well-deve- 

 loped, with the apex more or less free. The posterior nares are 

 protected by a soft palate, and the larynx by an epiglottis : the 

 rings of the trachea are generally cartilaginous and incomplete 

 behind : there is no inferior larynx. The oesophagus is continued 

 without partial dilatations to the stomach, which varies in its 

 structure according to the nature of the food, or the quantity of 

 nutriment to be extracted therefrom. 



The true vertebrae of Mammalia have their bodies ossified from 

 three centres, and present for a longer or shorter period of life a 

 discoid epiphysis at each extremity. They are articulated by 

 concentric ligaments with interposed glairy fluid forming what 

 are called the intervertebral substances ; the articulating surfaces 

 are generally flattened, but sometimes, as in the neck of certain 

 Ruminants, they are concave behind and convex in front : such 

 a vertebra, however, may be distinguished from a vertebra of 

 a Eeptile, with a similar ball-and-socket structure of the articular 

 surfaces, even when found in a fossil state, and when the test of the 

 articulating medium cannot be applied, by the complete anchylosis 

 or confluence of the annular with the central part or body, and 

 by the large relative size of the canal for the spinal chord. The 

 cervical vertebrae, with one or two exceptions, are seven in 

 number, neither more nor less : the Monotremes, which are the 

 instances commonly opposed to other generalizations, form no 

 exception to this rule. The lumbar vertebrae are more constant 

 and usually more numerous than in other classes of vertebrate 

 animals. The atlas is articulated by concave articular processes 

 to two convex condyles, which are developed from the ex- occipital 

 elements of the last cranial vertebra. The tympanic element of 

 the temporal bone is restricted in function to the service of the 

 organ of hearing, and never enters into the articulation of the 

 lower jaw. The olfactory nerves escape from the cranial cavity 

 through numerous foramina of a cribriform plate. The optic 

 foramina are always distinct from one another. 



The scapula is generally an expanded plate of bone ; the cora- 

 coid, with two (monotrematous) exceptions, appears as a small 

 process of the scapula. The sternum consists of a narrow and 

 usually simple series of bones : the sternal portions of the ribs are 

 generally cartilaginous and fixed to the vertebral portions without 



1* 



