line wide, but at the time of laying eggs it becomes more than two feet in length and nearly half an inch in width, thus increasing its volume about fifty times; and this remarkable change takes place annually.

II. In the Male, the Testes are a pair of whitish-yellow glands, of oval or globular shape-occasionally (as in Cypselus) vermiformand lie at the anterior end of the KIDNEYS, being kept in position by an enveloping peritoneal lamella, whence septa extend into the interior. Within the meshwork thus formed are embedded the spermatic vesicles or tubuli seminiferi, which combine toward the median side of each testis into wider tubes that in their turn leave it, and joining numerous convoluted canals, the whole constitute the Epididymis, which is irregular in shape and as a rule of a deeper colour. Generally the left testis is bigger than the right, although both are equally functional. During the breeding-season they are greatly enlarged, as has been most often remarked in the case of the House-Sparrow, where they increase from the size of a mustard-seed to that of a small cherry, temporarily displacing the usual arrangement of intestine, liver and stomach. The canals of each epididymis unite to form a narrow tube, the vas deferens, that, with small undulations, passes laterally along the ureter of the same side, over the ventral surface of the kidney, and opens upon a small papilla into the urodæum of the CLOACA (p. 90). The walls of the vasa deferentia are furnished with unstriped muscular fibre, but are devoid of glands, and there are no accessory glands, seminal or prostate. In many birds, especially the Passeres, the vasa deferentia increase considerably in length during the breedingseason, and form a closely convoluted mass which often causes a protrusion of the cloacal walls, a peculiarity that is particularly remarkable in some of the Ploceida,1 and has been observed in Accentor collaris.

The spermatozoa of Birds, though extremely minute, have a complicated structure, the different parts of which present so many differences of shape, size and proportion in various groups, that they may possibly afford characters of no mean taxonomic value (cf. Ballowitz, Anat. Anzeiger, 1886, pp. 363-376, and Arch. mikrosk. Anat. xxxii. pp. 402-473, tabb. 14-18).

RETINA, the visual or perceptive screen formed by the terminal expansion of the optic nerve and lining the inner chamber of the EYE.

1 The external protrusion thus caused in certain of the South-African Weaver-birds is often visible in their prepared skins, for it dries into a hard hook-shaped excrescence and has given rise to various absurd and speculative explanations.

RHEA, the name given in 1752 by Möhring1 to a SouthAmerican bird which, though long before known and described by the earlier writers-Nieremberg, Marcgrave and Piso (the last of whom has a recognizable but rude figure of it)—had been without any distinctive scientific appellation. Adopted a few years later by Brisson, the name has since passed into general use, especially among English authors, for what their predecessors had called the American Ostrich; but on the European continent the bird is commonly called Nandu,2 a word corrupted from a name it is said to have borne among the aboriginal inhabitants of Brazil, where the Portuguese settlers called it Ema (cf. EMEU). The resemblance of the Rhea to the OSTRICH was at once perceived, but the differences between them were scarcely less soon noticed, for some of them are very evident. The former, for instance, has three instead of two toes on each foot, it has no apparent tail, nor the showy wingplumes of the latter, and its head and neck are clothed with feathers, while internal distinctions of still deeper significance have since been dwelt upon by Prof. Huxley (Proc. Zool. Soc. 1867, pp. 420422) and the late Mr. W. A. Forbes (op. cit. 1881, pp. 784-787), thus justifying the separation of these two forms more widely even than as Families; and there can be little doubt that they should be regarded as types of as many Orders-Struthiones and Rhex-of the Subclass RATITE. Structural characters no less important separate the Rheas from the Emeus, and, apart from their very different physiognomy, the former can be readily recognized by the rounded form of their contour-feathers, which want the AFTERSHAFT that in the Emeus and CASSOWARIES is so long as to equal the main shaft, and contributes to give these latter groups the appearance of being covered with shaggy hair. Though the Rhea is not decked with the graceful plumes which adorn the Ostrich, its feathers have yet a considerable market-value, and for the purpose of trade in them it is annually killed by thousands, so that it has been already extirpated from much of the country it formerly inhabited, and its total extinction as a wild animal is probably only a question of time. Its breeding-habits are precisely those which have been

1 What prompted his bestowal of this name, so well known in classical mythology, is not apparent.

2 The name Touyou, also of South-American origin, was applied to it by Brisson and others, but erroneously, as Cuvier shews, since by that name, or something like it, the JABIRU is properly meant.

3 Ann. Nat. Hist. ser. 4, xx. p. 500.

Mr. Harting, in his and Mr. De Mosenthal's Ostriches and Ostrich Farming, from which the woodcut here introduced is by permission copied, gives (pp. 67-72) some portentous statistics of the destruction of Rheas for the sake of their feathers, which, he says, are known in the trade as “Vautour" to distinguish them from those of the African bird.

already described in the case of other Ratite birds. Like most of them it is polygamous, and the male performs the duty of incubation, brooding more than a score of eggs, the produce of several females-facts known to Nieremberg more than two hundred and fifty years since, but hardly accepted by naturalists until recently.

From causes which, if explicable, do not here concern us, no examples of this bird seem to have been brought to Europe before the beginning of the present century, and accordingly the descriptions previously given of it by systematic writers were taken at second hand, and were mostly defective if not misleading. In 1803 Latham issued a wretched figure of the species from a half-grown

specimen in the Leverian Museum, and twenty years later said he had seen only one other, and that still younger, in Bullock's collection (Gen. Hist. B. viii. p. 379).1 A bird living in confinement at Strasburg in 1806 was, however, described and figured by Hammer in 1808 (Ann. du Muséum, xii. pp. 427-433, pl. 39), and, though he does not expressly say so, we may infer from his account that it had been a captive for some years. In England the Report of the Zoological Society for 1833 announced the Rhea as having been exhibited for the first time in its gardens during the preceding twelvemonth. Since then many other living examples have been introduced, and it has bred both there and elsewhere in Britain, but the young do not seem to be very easily reared.2

6

Though considerably smaller than the Ostrich, and, as before stated, wanting its fine plumes, the Rhea in general aspect far more resembles that bird than the other Ratitæ. The feathers of the head and neck, except on the crown and nape, where they are dark brown, are dingy white, and those of the body ash-coloured tinged with brown, while on the breast they are brownish-black, and on the belly and thighs white. In the course of the memorable voyage of the Beagle,' Darwin came to hear of another kind of Rhea, called by his informants Avestruz petise, and at Port Desire on the east coast of Patagonia he obtained an example of it, the imperfect skin of which enabled Gould to describe it (Proc. Zool. Soc. 1837, p. 35) as a second species of the genus, naming it after its discoverer. Rhea darwini differs in several well-marked characters from the earlier known R. americana. Its bill is shorter than its head; its tarsi are reticulated instead of scutellated in front, with the upper part feathered instead of being bare; and the plumage of its body and wings is very different, each feather being tipped with a distinct whitish band, while that of the head and neck is greyish-brown. A further distinction is also asserted to be shewn by the eggs-those of R. americana being of a yellowish-white, while those of R. darwini have a bluish tinge. Some years afterwards Mr. Sclater described (op. cit. 1860, p. 207) a third and smaller species, more closely resembling the R. americana, but having apparently a longer bill, whence he named it R. macrorhyncha, more slender tarsi and shorter toes, while its general colour is very much darker, the body and wings being of a brownish-grey mixed with black. The precise geographical range of these three species is still undetermined. While R. americana is known to extend from Paraguay and southern Brazil through the state of La Plata to an uncertain distance in Patagonia, R. darwini seems to be the proper 1 The ninth edition of the Companion to this collection (1810, p. 121) states that the specimen "was brought alive" [? to England].

2 Interesting accounts of the breeding of this bird in confinement are given, with much other valuable matter, by Mr. Harting in the work already cited.

inhabitant of the country last named, though M. Claraz asserts (op. cit. 1885, p. 324) that it is occasionally found to the northward of the Rio Negro, which had formerly been regarded as its limit, and, moreover, that flocks of the two species commingled may be very frequently seen in the district between that river and the Rio Colorado. On the "pampas" R. americana is said to associate with herds of deer (Cariacus campestris), and R. darwini to be the constant companion of guanacos (Lama huanacus)-just as in Africa the Ostrich seeks the society of zebras and antelopes. As for R. macrorhyncha, it was found by Forbes (Ibis, 1881, pp. 360, 361) to inhabit the dry and open "sertoes" of north-eastern Brazil, a discovery the more interesting since it was in that part of the country that Marcgrave and Piso became acquainted with a bird of this kind, though the existence of any species of Rhea in the district had been long overlooked by or unknown to succeeding travellers.1

RHINOCEROS-BIRD, an old book-name for one or more of the HORNBILLS (p. 433), and occasionally used by modern SouthAfrican travellers for the Ox-PECKER (p. 680).

RIBS, if typically developed, have a double attachment to the vertebræ a capitulum or "head" articulating with the centrum of a vertebra, and a tuberculum or knob movably applied to the transverse process of the same vertebra. The portion next to the "head is known as the "neck," and to it succeeds the shaft, composed of two pieces, the dorsal or vertebral (to the posterior margin of which is generally attached an UNCINATE PROCESS) and the ventral, which is sometimes called the sternal or sterno-costal rib. If this ventral piece reaches and articulates with the sternum, the whole is called a "true" Rib; but if the sternum is not reached, the whole is called a "false" Rib, even if the ventral piece be present.

According to their position Ribs are usually distinguished as (1) Cervical Ribs possessing only a short shaft, while both head and tubercle are immovably fused with the vertebra; (2) Cervico-dorsal Ribs movably attached to the vertebræ, being in number from 1 to 4 on each side, with a shortened shaft which may in some cases carry a small ventral piece; (3) Thoracic Ribs, connecting the vertebral column with the sternum, from 3 to 9 in number-as

1 Beside the works above named and those of other recognized authorities on the ornithology of South America such as Azara, Prince Max of Wied, Prof. Burmeister and others, more or less valuable information on the subject is to be found in Darwin's Voyage; Dr. Böcking's "Monographie des Nandu" in (Wiegmann's) Archiv für Naturgeschichte (1863, i. pp. 213-241); Prof. R. O. Cunningham's Natural History of the Strait of Magellan and paper in the Zoological Society's Proceedings for 1871 (pp. 105-110), as well as Dr. Gadow's still more important anatomical contributions in the same journal for 1885 (pp. 308 et seqq.)