Air-breathing Vertebrata, with a skin remarkably deficient in glands and an epidermic skeleton in the shape of feathers, scales, or scutes. The skull articulates with the vertebral column by means of a single occipital condyle into which the basi- and ex-occipital bones enter in varying proportions. The pro-, epi-, and opisth-otic bones either remain separate inter se, and fuse with adjoining bones, or else fuse with adjoining bones and then with each other about the same time (Birds). There is an interorbital septum and the carotids pierce the basisphenoid, entering the cranial cavity at the pituitary fossa. The mandible is always complex: each ramus consisting of one cartilage bone, the articular, and five membrane bones, the dentary, splenial, coronoid, angular and surangular. It articulates with the skull by the intermediation of the upper part of Meckel's arch, the quadrate bone, which is free or fixed. There are well-developed cervical ribs, and the neck passes insensibly into the thorax.
The sternal elements of the ribs unite on each side in the embryo to form a broad plate on the right and left side of the body, which are always in contact and with rare exceptions fuse in the middle line: and either remain as cartilage more or less ossified, or are eventually replaced by membrane bone. When the pelvis is present, the true axis of the ilium trends forwards and downwards. There are as in all lower Vertebrata no epiphyses to the bones. There is no corpus callosum. The olfactory nerve forms a single strand invariably. The cervical sympathetic cord is generally double. The ciliary muscle of the eye is transversely striated and the cones of the retina contain clear and variously coloured oil globules except in Ophidia, the Crocodile and Geckoes. There is a cloaca (= proctodaeum) common to the rectum and urino-genital ducts. The heart has two auricles and either one or two ventricles, but the single ventricle is physiologically divisible into two. The haematids are oval and nucleated and smaller than in Ichthyopsida. The urine is semisolid and contains urates and not urea. The ova have a single layer of cells in the tunica granulosa of the Graafian follicles: they are large and projecting from the ovary when ripe.
The oviduct has entire edges as in all lower Vertebrata to its abdominal aperture, and is divisible into a narrower conducting portion, a glandular portion in which albumen is secreted, and a lower muscular or uterine portion in which the ovum stays while the shell is formed. Accessory glands to the reproductive organs are nearly invariably absent. The egg-shell is more or less calcareous. All are oviparous with rare exceptions among the Reptilia. The ovum is telolecithal and segmentation consequently partial.
There are two classes, Aves and Reptilia.