Xlll. 



into three portions, the anterior segment or presternum, the 

 posterior or xiphisternum, and a varying number between the two 

 called the mesosternum. In the Whalebone Whales ( Balcenidce) 

 only the presternum, supporting a single pair of ribs, is developed. 



Ribs. The ribs form a series of long, narrow, more or less 

 flattened bones, extending laterally from the sides of the vertebral 

 column and mostly joined directly or indirectly to the sternum. 

 Those which articulate directly are known as " true " ribs, and 

 always belong to the anterior portion of the series, while the 

 posterior ribs, which are either attached each to the edge of its 

 preceding rib, or are free, are called, in the former case "false," 

 in the latter, "floating " ribs. The portion of each rib nearest 

 to the sternum is usually imperfectly ossified or permanently 

 cartilaginous, and such parts are termed the "costal cartilages"; 

 in the rare cases in which they are completely ossified, as in the 

 Armadillos, they are known as "sternal ribs." 



Appendicular Skeleton. The appendicular portion of the frame- 

 work consists, when fully developed, of two pairs of limbs, an 

 anterior and a posterior. 



Anterior Limb. The anterior limb is present and fully de- 

 veloped in all Mammals, and is composed of a shoulder-girdle and 

 three segments of the limb proper, the upper arm (brachium), 

 the fore-arm (antibrachium), and the hand (manus). 



Shoulder-girdle. The shoulder girdle is in most Mammals in a 

 modified condition, compared with that in which it exists among 

 the four remainingclasses of Vertebrate Animals. In all Mammals, 

 except the Monotremes in which it is complete and articulated 

 with the sternum the coracoid is only present in the form of a 

 process or even minute tubercle. The blade-bone (scapula) is 

 always well developed, has a ridge on its outer surface the 

 so-called spine of the scapula which generally terminates exter- 

 nally in a free curved process called the "acromion." The form 

 of the scapula and the development of its processes largely 

 depends on the uses to which the limb is put, for instance 

 whether it be for burrowing, climbing, swimming, or merely for 

 the support of the body. The collar-bone (clavicle), an accessary 

 bar which connects the scapula with the sternum, may be present, 

 as in Man and all Marsupials, except the Bandicoots ( PeramelidceJ; 

 rudimentary and floating free in the flesh, as in the Dog, Cat, &c.; 

 or entirely absent, as in Seals, Whales, Ungulates, &c. 



Brachium and Antibrachium. The proximal segment of the 

 limb consists of a single bone, the humerus, and the second 

 segment of two bones, the radius and ulna, placed side by side, 

 and articulating with the humerus at their proximal, and with 

 the carpus at their distal extremities. In most Mammals these 

 bones cross each other, the radius in front of the ulna, so that the 

 former, though nominally in vertebrates external at the upper 

 end is internal at the lower. In the majority of Mammals the 



