BIRDS. 331 



A marked sauropsidan feature is found in the scales, which 

 more or less completely cover the feet and tarso-metatarsal 

 region. These may be small and numerous (reticulate tarsus) 

 or in larger plates extending across the tarsus from side to side 

 (scutellate), or the scutes on either side may fuse into a contin- 

 uous plate (booted tarsus). Various modiiications and combi- 

 nations of these conditions may occur. Dermal ossifications are 

 entirely lacking. 



The skeleton of birds is usually characterized by lightness 

 and strength, the former being attained by the hollow condition 

 of most of the bones, while strength is the result of frequent 

 fusion of parts which remain distinct in other vertebrates. 



In most birds the centra have saddle-shaped extremities. 

 Only in the odontormas were they amphicoelous, while in a few 

 water-birds the thoracic vertebrae are amphicoelous. The num- 

 ber of vertebrae varies greatly, the variations in number being 

 most noteworthy in neck and tail. The cervical region is long 

 and flexible, but in the rest of the column extensive fusions of 

 vertebrae take place ; the result being that usually the anterior 

 thoracic vertebras are coalesced, while the posterior thoracics, 

 lumbars, sacral (one to five in number), and some of the caudals 

 unite to form a synsacrum more or less intimately united with 

 the pelvis. The remaining caudals in existing birds are fused 

 to form a pygostyle or ploughshare bone ; but in the extinct 

 saururae the caudals were distinct and decidedly reptilian in 

 character. 



The ribs have two heads. Cervical ribs are usually present, 

 but are frequently fused with the vertebras. The thoracic ribs 

 are divisible into vertebral and sternal portions, the two parts 

 being articulated to each other without the intervention of a 

 cartilaginous portion. The sternal ribs join the sternum di- 

 rectly. Each vertebral rib bears on its posterior margin an 

 uncinate process which overlies the rib behind, thus giving 

 greater strength to the thoracic box.-' The sternum is well ossi- 

 fied, and has the shape of a broad plate, from the ventral side 

 of which in flying birds a strong keel or carina arises for the 

 attachment of the muscles of flight. The presence or absence 



^ Uncinate processes are lacking in a few birds {Chauna, Archaopteryx). 



