xiii. 
into three portions, the anterior segment or presternwm, the 
posterior or xiphisternum, and a varying number between the two 
called the mesosternum. In the Whalebone Whales ( Balenide) 
only the presternum, supporting a single pair of ribs, is developed. 
ftibs.—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 
Armadilios, 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 /brachiwm ), 
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 remaining classes 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, 
asin Man and all Marsupials, except the Bandicoots ( Peramelide/; 
rudimentary and floating free in the flesh, as in the Dog, Cat, é&e.; 
or entirely absent, as in Seals, Whales, Ungulates, ce. 
Brachium and Antibrachiwm.—The proximal segment of the 
limb consists of a single bone, the hwmerus, and the second 
segment of two bones, the radiws 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 
