GENERAL CHARACTERS OF BIRDS. 503 



which are light and airy. The curvature of the vertebral 

 centra is peculiar. The cervical vertebrae have little ribs. 

 A large number of vertebrse fuse with the two or three true 

 sacrals. The terminal vertebrae of the tail are fused in a 

 ploughshare bone. 



The bones of the skull fuse, the sutures being quite 

 obliterated. Only the lower jaw, the quadrate, the columella, 

 and hyoid, are always movable, but there may be a joint in 

 the beak at the end of the premaxillse. There is but one 

 condyle. A membrane bone called the basi-temporal covers 

 the basi-sphenoid. There is an inter-orbital septum formed 

 from presphenoid and mesethmoid. The otic bones fuse 

 with adjacent bones, and with one another about the same 

 time. There are no teeth. The jaws are covered by horny 

 sheaths. The premaxillae are large and form most of the 

 beak. The lower jaw consists on each side of five mem- 

 brane bones and a cartilage bone — the articular — which 

 works on the quadrate. 



There is a well-developed sternum, generally with a keel 

 to which the pectoral muscles are attached. The strong 

 coracoids reach and articulate with the sternum. In flying 

 birds, the clavicles are well developed, and are usually con- 

 nected by an interclavicle, which is often fused to the top of 

 the breast-bone. The fore-limb has not more than three 

 digits, the three metacarpals are fused, and there are only 

 two separate carpals, the others fusing with the metacarpals. 



The dorsal ilia of the pelvis are fused to the complex 

 sacrum ; the acetabulum is not thoroughly ossified ; the 

 pubes (or post-pubic processes) are directed backwards 

 parallel to the ischia. There is no pubic symphysis except 

 in the African ostrich (StrutMo\ and no ischiac symphysis 

 except in the American ostrich {Rkea). In the hind- 

 limb, the fibula is incomplete and united to the side of the 

 tibia; there are no free tarsal bones, half of them being 

 united to the distal end of the tibia (which is therefore 

 called a tibio-tarsus), the others being united to the proximal 

 end of three united metatarsals (which thus form a tarso- 

 metatarsus) ; the maximum number of toes is four, but if 

 there be a fourth, its metatarsal is free from the other three. 



In regard to the alimentary system, the absence of teeth, 

 the frequent occurrence of a crop and a gizzard, the frequent 



