314; PALEONTOLOGY, 



The presence of a clavicle is always associated with certain 

 uses of the anterior extremity ; thus monkeys, rodents, and 

 carnivora possess this bone in common with man, but it is 

 absent in ruminants and pachyderms. The arm is formed 

 of a single bone, the Tiumerus ; the fore-arm of two bones, the 

 radius and ulna ; the hand is formed of two ranges of bones, 

 the carpal and meta-carpal, and the pJialanges of the fingers ; 

 the thumb has two phalanges, and the other fingers three. 

 The posterior extremities (the cetacea excepted, in which 

 they are absent) are attached to the spinal column by 

 a bony arch, or pelvis, formed of three pairs of bones, ilia, 

 ischia, and pubes, which are separate in early life, but are 

 firmly soldered together in adult age. The thigh, like the 

 arm, consists of a single bone, the femur ; the leg, like the 

 fore-arm, of two bones, the tibia and the fibula ; the foot, 

 like the hand, of three ranges of bones, tarsal, meta-tarsal, 

 and phalanges. 



The jaws are for the most part armed with teeth, which 

 present great varieties as to form, structure, and arrangement 

 in the different orders; the teeth are very dense in this 

 class, and are the parts of the skeleton which resist destruc- 

 tion longest ; they are consequently the best preserved : the 

 paleontologist, therefore, studies their form and microscopic 

 structure with great attention, as the teeth often remain in a 

 high state of preservation after the bones have been decom- 

 posed or water-worn. Some mammals are preserved entire 

 by the antiseptic power of ice, as the rhinoceros and elephant 

 of Siberia, in which even the muscles and the ligaments, 

 integument and hair, were preserved. 



We arrange the class into two divisions : the DLDELPHIA 

 and the MONODELPHIA. 



The DIDELPHIA, by a certain organic disposition, are born 

 in an immature state at an early period of their development, 

 and require special organs for their preservation, consisting 

 of* a pouch in the abdomen of the mother, in which are 

 situated the nipples of the milk glands. The young adhere 

 to these organs during the first period of their pouch-life, 

 and never quit the mammae until their development has 

 advanced. The didelphia have special bones, called marsu- 

 pial, for supporting the pouch. The kangaroos, and other 

 Australian animals, belong to this class. 



