The sexes differ much externally in colour, development of peculiar feathers, etc, and in Accipitrine birds the females, as is so commonly the case in Arthropoda, are larger than the males. The testes are always retained within the abdomen, and lie anteriorly on the kidneys. They undergo a periodical enlargement at the breeding season and the left is occasionally the larger of the two. The vasa deferentia lie to the outer side of the ureters, and when filled with spermatozoa are disposed in short wavy folds. They are slightly dilated at their cloacal terminations. The right ovary and oviduct are usually atrophied, and when the ovary is persistent, as in some Accipitres (=Aetomorphae), its ova do not come to maturity. The oviducal aperture is very wide in correlation with the large size of the ova. The length of the duct and the number of its coils increase with the breeding season. It is divisible into three sections: the first narrow, the second glandular and secreting the albumen, the third muscular and glandular. This last part is often termed 'uterus,' as the egg stays in it for some time whilst the calcareous shell and its colouring matter, if any, are secreted by the glands. There are no accessory glands appended to the generative ducts in either sex.

The Ostrich has a solid grooved intromittent organ similar to that of Chelonia and Crocodilia: the Duck and other aquatic birds, the remaining Ratitae and a few others, an eversible grooved organ attached to the front wall of the cloaca. The organ is represented in the female. The egg is impregnated in the upper part of the oviduct or on the ovary. It is incubated by the female, in rare instances by the male as well or exclusively, and is generally laid in a special nest or shelter constructed for the purpose. Development lasts for a time dependent on the size of the bird, e.g. about II days in smaller birds to 7 weeks in the Ostrich. During the growth of the young bird an air space is formed and gradually increases in size at the obtuse end of the egg. Many young birds are provided with a hard knob on the upper surface of the bill for breaking through the shell when ready for hatching. When the food-yolk is large, the young are hatched well-clothed with downy feathers, able to run or swim and provide for themselves, e. g. gallinaceous birds, waders, Lamellirostres, Ratitae. Such birds are termed Praecoces or Autophagi. But when the food-yolk is small in amount, the young are hatched, either naked or with little down, unable to run or swim, and requiring to be fed by the parents and to be brooded on by the female: e. g.

Accipitrine birds, Passeres, and many others. Such birds are termed Altrices or Insessores. Most of them are monogamous and have few young, while many of the Praecoces are polygamous and their young numerous.

The classification of Birds presents great difficulties, and many arrangements have been proposed which it is impossible to discuss in a short compass. Three chief divisions are generally recognised - Saururae, Ratitae, and Carinatae. The first contains the Jurassic Archaeopteryx, the second a few living Birds mostly of large size, such as the Ostrich, Cassowary, Emu, etc, incapable of flight and devoid of a keel to the sternum; the third all Birds capable of flight and with a keel to the sternum. Prof. Damés in a recent memoir (Palaeont. Abhandl. Berlin, ii. part 3, 1884) has proposed to classify Archaeopteryx in the group of Carinatae. His views have been criticised by Paulow in the Bulletin de la Soc. Imp. des Naturalistes de Moscou, 60, p. 100, 1884, who has pointed out that in several respects Archaeopteryx can hardly be considered as within the line of descent of Carinatae. Retaining the Saururae as a separate group but following Damés in other points, the classification given below may be adopted as showing the main outlines.

For subordinate divisions and discussions on the various systems proposed, the student must refer to an article on 'Ornithology' by Prof. Newton in the Encyclopaedia Britannica (ed. ix.) xviii. 1885.

I. Saururae

Vertebrae biconcave; sternum broad (? a keel), well ossified; abdominal ribs; tail long with separate caudal vertebrae (i. e. no ploughshare bone); three fingers with separate metacarpals, all clawed; pelvic bones separate; fibula complete, its distal end in front of tibia; metatarsals united but not to the degree observable in living birds. Archaeopteryx from the Solenhofen Slates.

II. Ratitae

Feathers of the adult with free barbs; an after-shaft to those of the body; sternum devoid of keel; anterior limb shortened or rudimentary; incapable of flight; teeth when present lodged in a groove (Hesperornis).

(1) Imperfectly Known Order

Laopteryx from Jurassic strata in America.

(2) Odontolcae (Marsh)

Rami of lower jaw separate; wing with only a humerus. Vertebrae with typical avian centra; no ploughshare bone. Hesperornis from American Cretaceous strata.

(3) Post-Cretaceous Ratitae

Edentulous; rami of lower with anchylosed symphysis. Vertebral centra typical; wings rudimentary but with humerus, fore-arm and rudimentary hand. Tertiary, Diluvial and living: includes the living Apteryx, Cassowary, Emu, Ostrich and Rhea, and the fossil Aepyornis from Madagascar, Dinornis (Moa) and Palapteryx from New Zealand.

III. Carinatae

Contour feathers in the adult; sternum with a keel; anterior limb well developed; capable of flight; teeth, when present, lodged in sockets (Ichthyornis).

(1) Odontotormae (Marsh)

Vertebrae biconcave; rami of lower jaw not anchylosed; tail short; some of the vertebrae fused. Ichthyornis from American Cretaceous strata.

(2) Post-Cretaceous Carinatae

Adult edentulous, but the fossil Argillornis longipennis has dental alveoli; rami of lower jaw anchylosed; vertebral centra typical; tail short; a ploughshare bone. Tertiary, Diluvial and living. For the literature of Fossil Birds, see p. 66, ante.

Teeth in Parrots, Fraisse, Verhandl. Phys. Med. Gesellsch. Wurzburg, xv. 1881, SB. p. iii.

Classification, Newton, 'Ornithology,' Encyclopaedia Britannica (ed. ix.) xviii. 1885; cf. Huxley, P. Z. S. 1867-1868 j Sclater, The Ibis, 1880; and collected papers of Garrod and Forbes.