660 BIRDS. 
vical, thoracic, lumbar, sacral, and caudal. The mobile 
neck consists of fourteen cervical vertebre ; from the third. 
to the twelfth these bear short ribs fused to the centra and 
transverse processes ;: the thirteenth and fourteenth have 
them free and well developed, but not reaching the sternum. 
-Of the thoracic vertebra, namely, those whose ribs reach 
the sternum, the anterior three are fused to one another, 
while the fourth is free. The complex sacral region consists 
of the fifth thoracic (with free ribs reaching the sternum), 
five or six lumbars, two sacrals, and five caudals, all fused. 
Lastly, there are six free caudals ending in a pygostyle or 
ploughshare bone,—a fusion of about four vertebre (cf. 
coccyx in man). This bone serves as a base for the 
rectrices. 
A cervical vertebra shows on the anterior surface of the 
centrum a distinctive curvature, described as saddle-shaped 
or heteroccelous. It is concave from side to side, convex 
from above downwards. Posteriorly the curvatures are, of 
course, the reverse. 
The ribs have two heads—a capitulum articulating with 
a centrum, a tubercle articulating with a transverse process. 
The ventral part of the rib, which reaches the sternum, is 
called the sternal rib, and is joined at an angle to the dorsal 
part, which articulates with a vertebra. In Birds the sternal 
ribs are always bony ; in Mammals they are usually cartila- 
ginous. On the posterior surface of each of the first four 
thoracic ribs there is an uncinate process, absent only in 
the 5. American screamers (Palamedez). 
The skull has a rounded cranial cavity, large orbits, and 
a narrow beak, which is mostly composed of the premaxillz. 
All the bones are fixed except the quadrate, lower jaw, 
columella, and hyoid. The surface is polished ; the sutures 
are obliterated very early in life. 
The back part of the skull is formed by the basioccipital, 
the two exoccipitals, and the supraoccipital, surrounding 
the foramen magnum. The basioccipital forms most of the 
single condyle. 
‘The roof of the skull is formed from the paired parietals, 
frontals, and nasals, the last being small and in part super- 
seded by the upward extension of the premaxillz. 
The line of the upper jaw consists of premaxilla, small 
