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enlarged in the breeding season; in the sparrow, for instance, from the size of a mustard seed to that of a small cherry. The vas deferens descends with many undulations down the lateral side of the ureter of the same side, and opens upon a small papilla into the urodaeum. Extraordinary increase in length during the breeding season causes the vasa deferentia in some of the African weaver-birds to protrude, or to bulge out the cloacal walls beyond the vent. The spermatozoa exhibit many differences in shape, size and proportions, in the various groups of birds. They have been studied minutely by E. Ballowitz.

Only the left ovary becomes functional, with rare individual exceptions. Both present the appearance of diminutive clusters of grapes, at the anterior end of the kidneys, close to the suprarenal bodies, separated from each other by the descending aorta and by the vena cava where this is formed by the right and left vena iliaca communis. During the breeding season many more eggs are developed than reach maturity, amounting in most birds to several dozens. Those germs which do not ripen during the season undergo a process of resorption, and in the winter the whole ovary dwindles to often a diminutive size. In young birds both oviducts are almost equal in size, but the right soon degenerates into an insignificant strand. During every laying season the left duct increases enormously by new formation of its component fibres. For instance, in the fowl its volume increases about fifty-fold, growing from some 6 in. in length and scarcely one line in width to more than 2 ft. in length and ½ in. in thickness. The upper, wide opening of the duct is attached by elastic, peritoneal lamellae to the hinder margin of the left lung; the middle portion of the duct is glandular and thick-walled, for the deposition of the albumen; it is connected by a short, constricted “isthmus” (where the shell-membrane is formed) with a dilated “uterus” in which the egg receives its calcareous shell and eventual pigmentation.

Bibliography.—A. v. Brunn, Rückbildung nicht ausgestossener Eierstockseier, Henle Festschrift (Bonn, 1882); E. Ballowitz, “Die Spermatozoen der Vögel,” Arch. Mikr. Anat. xxxii., 1888, pls. 14-18; M. Sacchi, “Contribuzione all’ istiologia del ovidotto dei saurop-sidi,” Att. Soc. Ital., Milano, vol. xxx.; W. A. Forbes, “On the Bursa Fabricii in Birds,” P.Z.S., 1877, pp. 304-318; H. Gadow, “Remarks on the Cloaca and on the Copulatory Organs of the Amniota.” Phil. Trans., 1887, pp. 5-37, pls. 2-5; Martin Saint Ange, “Étude de l’appareil reproducteur dans les cinq classes d’animaux vertébres,” Mem. Ac. Soc., Paris, xiv., 1856; E. Retterer, “Contribution à l’étude du cloaque et de la bourse de Fabricius,” Robin’s Journ. de l’anat. et physiol., 1885, pp. 369-454, pls. 17-19.

B. Fossil Birds

Much had naturally been expected from the study of fossil birds, but, so far as the making of classifications is concerned, they have proved rather a source of perplexities. So long as the characters of new fossils are only of specific and generic value, it is mostly possible to assign the birds to their proper place, but when these characters indicate new families or orders, for instance Hesperornithes, Ichthyornithes, Palaelodi, their owners are put outside the more tersely constructed classifications applicable to modern birds. It is no exaggeration to say that the genus, often even the species, can be determined from almost any recent bone, but in the case of Miocene, and still more, of Eocene fossils, we have often to deal with strange families, which either represent an extinct side branch, or which connect several recent groups with each other. Our artificially-established classifications collapse whilst we gain further insight into the mutual affinities of the existing groups. Of course this must be so if evolution is true. But it also follows that, if every extinct and recent bird were known, neither species, nor genera, nor families, nor orders could be defined. We should be able to construct the pedigree of every group, in other words, the gigantic natural system, but there would be no classification. Much light has also been thrown by fossil birds upon the study of geographical distribution. The key to the distribution of recent groups lies in that of the extinct forms. Not only have many absolutely new families been discovered, but many kinds of modern birds are now known to have existed also in countries which they are now extinct. There were, for instance, trogons, secretary-birds, parrots, and other now Ethiopian forms in Miocene France. Ostriches, undistinguishable from Struthio, have been found in Samos and in the Sivalik Hills.

The proper study of fossil birds may be said to have begun with A. Milne-Edwards, whose magnificent Oiseaux fossiles de la France was published from 1867 to 1871. This work deals chiefly with mid-Tertiary forms. A new impetus was given by O. C. Marsh, who, after 1870, discovered a great number of bird remains in the Cretaceous strata of North America. The most important result is the proof that, until the end of the Cretaceous epoch, most, if not all, birds were still possessed of teeth (see Odontornithes).

The oldest known bird is the Archaeopteryx (q.v.), of the upper Oolite in Bavaria. The imprints in the enormously older new red sandstone or Lower Trias of Connecticut, and originally named Ornithichnites, belong to Dinosaurian Reptiles.

A wide gap separates Archaeopteryx from the next order of fossil birds of the Cretaceous epoch, and, since freshwater deposits of that age are rare, bird remains are uncommon. Many bones formerly referred to birds have since proved to belong to Pterodactyls, e.g. Cimoliornis from the English Chalk. But in 1858 were discerned in the Upper Greensand of Cambridgeshire remains which are now known as Enaliornis. W. Dames has described bones from the Chalk of southern Sweden under the name of Scaniornis, probably allied to Palaelodus. From the Cretaceous rocks of North America a large number of birds have been described by O. C. Marsh. Of these the most interesting are Ichthyornis (= Graculavus) and Hesperornis, from the Cretaceous shales of Kansas. They were placed by Marsh in a distinct subclass of birds, Odontornithes (q.v.). Probably all birds of Cretaceous age were still possessed of teeth. Baptornis, another of Marsh’s genera, seems to be allied to Enaliornis, Palaeotringa and Talmatornis, were by him referred to Limicoline and Passerine birds. Laornis from the Cretaceous marls of New Jersey was as large as a swan.

1911 Britannica-Bird- Head of Odontopteryx1.png
Fig. 17.—Remains of head of Odontopteryx, from the original in the British Museum; side view; natural size.

1911 Britannica-Bird- Head of Odontopteryx2.png
Fig. 18.—Remains of head of Odontopteryx, seen from above.

The lower Eocene has furnished a greater number of bird bones. Some of the largest are those of Gastornis, with three species from France, Belgium and England. Much difference of opinion obtains as to the affinities of these birds, which were far larger than an ostrich; they were undoubtedly incapable of flight and there are indications of teeth in the upper jaw. Provisionally this genus has been grouped with the Ratitae, which at any rate are a heterogenous assembly. Sir R. Owen’s Dasornis, of the London Clay, known from an imperfect cranium, and E. D. Cope’s Diatryma of New Mexico, based upon a gigantic metatarsus, may also belong there. The London Clay of South England has likewise supplied some long upper arm bones, Argillornis. The most remarkable specimen is a skull, Odontopteryx toliapicus (figs. 17, 18); the edges of the jaws were serrated like those of certain tortoises. The character of this skull and the compound rhamphotheca (known by the imprints left upon the jaws) indicate affinities with the Steganopodes. Remnants