MAMMALIA. 637 



Mammiferous lower jaw, ought to be well remembered by the 

 geologist. 



We shall hereafter have occasion to describe the teeth that arm 

 the jaws of the different tribes of quadrupeds ; and therefore now 



Fig. 295. 



proceed to examine their cranial cavity, and the bones that enter 

 into its formation. 



The frontal bones (Jigs. 294, 295, 1, 1) are generally two in 

 number ; and even when, as in Man, they seem to form but one 

 bone, the two lateral halves are produced from separate points of 

 ossification, and only coalesce as age advances: sometimes, indeed, 

 even in the adult, they remain permanently separated by suture. 



The parietal bones (7, 7) occupy their usual position ; and, 

 although generally double, as in the human skeleton, they are not 

 unfrequently consolidated together, even at an early age, so as to 

 represent but a single bone. 



The occipital bone consists primarily of the same pieces as in the 

 Reptile ; but in the Mammifer these are at an early period conso- 

 lidated into one mass, situated at the back of the cranium. Its 

 basilar portion (5) articulates with the atlas by two condyles ; while 

 the lateral wings (10) and the superior arch (8) surround the 

 foramen magnum, and protect the cerebellic regions of the 

 encephalon. 



The sphenoid (6), although composed of fewer separate pieces 

 than in the Reptilia, and even regarded by the human anatomist 

 as a single bone, is still distinctly divisible, especially in young 

 animals, into two very important portions, one anterior, and the 

 other posterior ; each, as we shall soon see, forming the body of a 

 distinct cranial vertebra. The posterior half (6) consists of the 



