8 MAMMALIA. 



Cheiroptera, Solidungula, and Pachydermata, and some few genera 

 from the other orders, of a single piece, as in Man, both halves hav- 

 ing become united in the median line at an early period before or 

 after birth. In other animals both the halves remain permanently 

 separated, and are held together only by ligamentous fibres. The 

 lower jaw is in its simplest condition in Balsena, where it resem- 

 bles a rounded arched rib. In the Dolphin it is somewhat deeper, 

 and provided with a small coronoid process. The ascending ramus 

 of the jaw, which is more or less conspicuous in the higher orders 

 of Mammalia, is frequently altogether wanting, as in Orycteropus. 

 The Carnivora have a strong and broad, the lluminantia, as the 

 Camel, a long and small, coronoid process. The lower border of 

 the symphysis is in Man alone curved forward and upward, in all 

 the Apes it slopes downward and backward. Many Mammalia, 

 such as the Carnivora and Rodentia, have a process directed back- 

 ward from the angle of the jaw, a structure which is very generally 

 met with in Birds. The form of the articulating condyle is subject 

 to great diversities, which usually characterize entire orders. Thus 

 it is very small, and plays freely in all directions within a shallow 

 glenoid cavity in the Ruminantia ; it is much elongated transversely. 

 and locked in a deep cavity of a corresponding form in the Car- 

 nivora so as to admit of no lateral motion ; it is lengthened out 

 from back to front, and chiefly moveable in this direction in the 

 Rodentia. 



Viewed as a whole, the form of the skull departs most from that 

 of Man in the lowest orders. Thus in the Cetacea the jaws are 

 generally lengthened out in the shape of a snout ; and the cranial 

 bones are united merely by squamous sutures. A want of lateral 

 symmetry occurs also in this order. In the Physeter the right 

 nasal orifice is much tl\e larger, the nasal partition is pushed to the 

 left side, and the nasal bones lie rather behind than by the side of 

 each other. In the Dolphin this asymmetrical condition is extended 

 to other bones, namely, the intermaxillaries. In the Narwhal the 

 lower jaw itself is asymmetrical, the left half like the correspond- 

 ing half of the upper jaw being the larger and broader. The skull 

 of the Monotremata ( Ornithorynchus, Echidna) is very bird-like 

 through the early coalescence of its bones, and the snout-shaped 

 jaws. In the higher orders the facial and maxillary bones con- 

 stantly retreat farther backward. In the Horse the facial & iour 

 times larger than the cranial portion of th* ^- a proportion 

 exactly the reverse of that of Man. Tiie depressions within the 



