SA UROPSIDA : A VES. 373 



CLASS AVES. 



Warm-blooded Sauropsida in which the epidermis developes feathers 

 clothing the head, neck^ body, fore-limbs and the upper part of the hind-limbs 

 (cf.pp. 51-2). The fore-limbs are modified into wings. The sacral series of 

 vertebrae is long and includes 1-3 dor sal > all the lumbar ; the two true sacral 

 and a variable number of caudal ( = urosacral) vertebrae^ all anchylosed 

 together. The bones of the pelvis are anchylosed : the ilium of great antero- 

 posterior extent: and the pub es (postpubes) and ischia do not meet in a ven- 

 tral symphysis. There is a carpo-metacarpus ; a tibio-tarsus and tarso-meta- 

 tarsus formed by the union of both proximal and distal tarsalia to the tibia and 

 metatarsals respectively. The jaws are covered by a horny epidermic sheath^ 

 and there is a more or less muscular gizzard in the digestive tract. The heart 

 is quadriloctdar and the right auriculo-ventricular valve muscular - , consisting 

 of two flaps. The aorta is single and crosses the right bronchus. The lungs 

 are firmly fixed to the back of the thorax, and there are air-sacs developed 

 from the ends of certain of the bronchia. The right ovary is atrophied. 



Epidermic scales are found covering the tarso-metatarsus and the toes, 

 but these parts may in exceptional cases be feathered. There are claws 

 on the last phalanges of the toes, frequently on the thumb and first finger. 

 A thickened epidermis covers the upper and lower jaw. It is subject to a 

 partial moult and renewal every year in the Puffin ; see Zoologist, 1878, 

 p. 233. There is no dermal exo-skeleton unless the bony spur on the 

 metacarpus in a few birds, e. g. Megapodius, the Mound-bird, and on the 

 posterior aspect of the tibio-tarsus in many Gallinae ( = Alectoromorpkae) is 

 to be considered a dermal structure. The corium or derm is remarkably 

 thin, very vascular, rich in Pacinian bodies, and contains numerous bundles 

 of non-striated muscle fibres connected to the follicles of the feathers. 

 There is but one skin-gland, the uropygial or oil gland, situated on the 

 dorsal aspect of the tail. 



The leading features of the skeleton are as follows 1 . The surface of 

 the skull is polished, and the sutures between the bones obliterated at an 

 early period ; the interorbital septum well ossified. The praemaxillae are 

 very large and the maxillae small, their palatal plates variably developed. 

 The parasphenoid is very large in the embryo and ossifies as an anterior 

 azygos bone, the rostrum, and posterior paired bones, the basi-temporals. 

 The quadrate is free, and usually articulates by two heads, both with the 

 cranium and the lower jaw : it is connected to the maxillae by jugo-quadrato- 

 jugal bones, the rami of the lower jaw are anchylosed at the symphysis 

 at an early period, and the constituent bones are also anchylosed. The 

 hyoid arch is rudimentary, but the first branchial well-developed. The 



1 For detailed description see Preparation ii. pp. 58-67. 



