and appeared to be strengthened by the occasional connection by ligament and bone with the hypocleidium.

The most recent modification of this view is put forward by PARKER, who has shown in Opisthocomus a needle-shaped splint of bone lying upon the keel, and therefore independent of it.

The development of the sternum throws a light upon the homologies of its different parts in different birds, and. in other vertebrates.

It is plain in the first place that the spina externa and the spina interna have nothing whatever to do with the manubrium sterni of the mammal; for they are (in the bird) secondary outgrowths, and not, as in the mammal, part of the primitive sternum formed by concrescence of the ribs. The same holds good of the posterior median region of the bone, which is a secondary outgrowth, and can therefore have no relations with the xiphoid process of the mammalian sternum; what does correspond to the latter are the posterior lateral processes of the avian sternum.'

Pelvis. The pelvis consists of three pairs of bones, the ilium, ischium, .and pubis. In the young embryo these bones form a continuous sheet of cartilage, but are all separate distally; the ilium is directed in an antero-posterior

The literature of the sternum is large, and is to a considerable extent to be found under the several groups. Memoirs of a wider scope are W. K. PARKER, A Monograph on the Structure and Development of the Shoulder Girdle and Sternum in the Vertebrata,' Ray Soc. Publications, 1868; L'HERMINIER, Recherches sur la Marche d'Ossifications,' &c., Mém. Ac. Sci. 1830. The history of the development of knowledge concerning the ossification of the sternum and the classificatory results therefrom is treated by NEWTON in Dict. Birds, Introduction.' Miss LINDSAY'S paper, already quoted, contains references to the chief memoirs upon the subject. See also R. DIECK, De Sterno Avium, Diss. Inaug., Halæ, 1867.

C. GEGENBAUR, Beiträge z. Kenntniss des Beckens der Vögel,' Jen. Zeitschr. vi. p. 157; MEHNERT, Untersuchungen über die Entwicklung des Os pelvis d. Vögel,' Morph. J.B. xiii. 1888, p. 259; A. JOHNSON, On the Development of the Pelvis Girdle, &c., in the Chick,' Quart. J. Micr. Sci. 1883, p. 399; B. HAIJ, Jemförande Studier öfver Foglarnes Backen, Lund, 1887, and 'Morphologisk Byggnoden af Ilium,' &c., Act. Lund. Univ. xxii. 1887, p. 1; G. BAUR, 'Bemerkungen über das Becken d. Vögel v. Dinosaurier,' Morph. J.B. x. 1885, p. 613; A. BUNGE, Untersuchungen zur Entwicklungsgeschichte des Beckengürtels, &c., Diss. Inaug., Dorpat, 1880.

direction; the ischium and the pubis look downwards and slightly backwards; at the end of the pubis, near to where it comes into contact with the iliac portion of the cartilage, is a forwardly directed process, the prepubic process. This primitive state of affairs has been most nearly preserved in Apteryx and Dinornis; in these birds the pubis and ischium are free from each other distally and from the ilium; their direction is, however, more backwards than in the embryo, and the prepubic process is relatively smaller. In all other birds the pubis and the ischium lie in a line more parallel

il, ilium; p, pubis; ps, interobturator process; lp, pectineal process.

with the ilium, and there is a greater or less connection between the several bones. This is seen in a less developed condition in the struthious birds and tinamous than in any others. In the tinamous, in fact, the pubis and ischium are quite free from each other distally, and from the ilium. In the ostrich the pubes unite in a ventral symphysis; in Rhea there is a remarkable modification induced by the meeting of the ischia. The details of the struthious pelvis will be found described under the description of that group. In carinate birds the ischium is for the greater part of its extent fused with the ilium, a foramen only-the ischiadic foramen -being left anteriorly. The pubes join to a less extent with the ischia, and are sometimes (e.g. Colymbus) quite free

from them. The prepubic or pectineal process is large in the struthious birds, in the tinamous, and in a few carinates, such as Geococcyx. It has received much attention as the possible equivalent of the reptilian pubis, the bird's so-called pubis being in that case the homologue of the posterior pubic process of the reptile. The dinosaurians seemed at one time to have been the means of solving the questions involved; for in some of them there is a backwardly directed pubis, not quite so bent as in the bird, from the anterior and upper end of which a stout bone, considered to be the homologue of the pectineal process, is directed forwards. This latter, however, is clearly a part of the pubis, while the pectineal process is at least not always a product of the pubis, being sometimes purely iliac in origin, sometimes partly pubic and partly iliac. It may be that the missing prepubic process of the dinosaurian pelvis is represented by the remarkable bones ankylosed to the ostrich's pelvis anteriorly, and continued forwards by a cartilaginous tract, which were discovered by DARWIN and GARROD. This will reduce the pectineal process to the level of a mere projection of the pelvis of no particular significance save as a secure hold for the important ambiens muscle which is there attached. In any case it is safe to assert that wherever that process is present and long the ambiens is also present and well developed.

Among carinate birds the pelvis does not show a great variability of form. The pelvis is either broader or narrower, being excessively narrow in the Colymbi. The proportions of the preacetabular and the postacetabular regions also differ, as do their relative breadths.

It is noteworthy that the pubis is sometimes defective in the middle, appearing then to consist of a proximal and distal portion unconnected in the dry skeleton. The fact also that but few muscles arise from the pubis seems to show that it is in a condition of degeneration.

In Archæopteryx alone are the elements of the pelvis not ankylosed together.

The acetabulum of all birds except Dromaus is perfo

rated; but the perforation is reduced in size in Geococcyx, Tinamus, and Hesperornis.

Hind Limb. The hind limb of birds consists of femur, tibia and fibula, tarsus, metatarsus, and phalanges.

In all birds the femur is shorter than the tibia, the proportions varying much. It seems impossible to place those birds in which the difference is least at the base of the series. on account, of course, of a resemblance so far to reptiles, since relative importance of the fore and hind limb appears to have something to do with the matter. From FÜRBRINGER'S tables it is to be gathered that Fregata is the bird in which the difference between those two segments of the leg is least. It is most pronounced in the divers, flamingo, and Tubinares. A bone of some classificatory importance is the patella,2 a sesamoid on the upper surface of the knee. This bone is not ossified at all in Colymbus, but is enormous in the grebes and in Hesperornis, in which latter, as in Phalacrocorax and Biziura lobata, it is perforated by the tendon of the ambiens. In Plotus the patella is grooved only for this tendon.

In no bird (except as an occasional abnormality) is the fibula a complete bone. It fails below, and does not reach the tarsus. It is usually more or less coalescent with the tibia. The latter is a strong bone with a crest in front, which is enormous in the divers and Hesperornis. The distal end of the tibia is formed by a portion of the tarsus, of which the remaining portion is coalesced with the metatarsus. The tarsus in the embryo 3 consists of three chondrites, a tibiale, a fibulare, and a distale. The latter represents the separate distal elements of the tarsus fused. The tibiale sends up

,

1 KESSLER (Osteologie der Vogelfüsse,' Bull. Soc. Nat. Mosc. 1841) has also given tables. The value of the long bones' of the leg for defining orders, families, and often genera' is plainly set forth in this paper. See also MILNEEDWARDS, Oiseaux Fossiles de la France, where further information is to be found. 2 J. KACZANDER, 'Beitrag zur Lehre über die Entwicklungsgeschichte der Patella,' Mt. Embr. Inst. Wien, (2) ii. 1887, p. 12.

3 G. BAUR, Der Tarsus der Vögel u. Dinosaurier,' Morph. J.B. viii. 1883, p. 417; E. G. MORSE, On the Carpus and Tarsus of Birds,' Ann. Lyc. New York, x. 1873, p. 141, and 'On the Identity of the Ascending Process of the Astragalus,' &c., Anniv. Mem. Bost. Soc. Nat. Hist. 1880.

wards an ascending process,' found also in the dinosaurs, which is the equivalent of the intermedium, while the centrale is represented by a distinct osseous nodule in the adults of the Struthiones (including Dinornis) and tinamous. In Apteryx T. J. PARKER found two osseous centralia.

The number of toes and phalanges has been already described above.

The Skull.'-While presenting many characteristic features of its own, the skull in birds shows certain fundamental likenesses to the skull of the reptilia. As in them, and contrary to what we find in the mammalia, the skull of birds

1. Articulates with the spine by a single occipital condyle.

2. Possesses a quadrate bone for the articulation of the mandible.

3. The mandible itself is composed of at least a dentary angular and articular portion.

4. The columella auris is very similar.

The bird's skull is, however, distinguishable by a number of characters, of which the following are the most important :

1. The bones of the cranium are very closely united and fused, this being less marked in the penguins and ratites. 2. The brain case is large as compared with that of reptiles.

3. The bones of the skull, as are those of the skeleton in general, are light and contain air spaces.

4. The columella and the os transversum of the reptiles are absent.2

5. There is no distinct postfrontal.

The bones of the bird's skull, as that of other vertebrates, may be distinguished into four categories(1) those of the cranium ossified from its cartilage; (2) those of the sense capsules; (3) those of the visceral arches, and (4) membrane bones connected with the several regions enumerated.

H. MAGNUS, 'Untersuchungen über d. Struktur d. knöchernen Vogelkopfes, Zeitschr. wiss. Zool. xxi. 1871.

2 See however below, under Passerine skull.