CLASS. AVES[94].
Birds form a large and extremely homogeneous class of the vertebrata,
and are readily distinguished from all other animals by the possession
of an epidermal exoskeleton having the form of feathers. Feathers
differ from hairs in the fact that they grow from papillae formed of
both the horny and the Malpighian layer of the epidermis, which
papillae at first project from the surface, and only subsequently
become imbedded in pits of the dermis. A dermal exoskeleton does not
occur in birds.
The endoskeleton is characterised by its lightness, the large bones
being generally hollow; but the pneumaticity does not vary in
proportion to the power of flight. The cervical part of the vertebral
column is very long and flexible, while the post-cervical portion is
generally very rigid, owing to the fusion of many of the vertebrae,
especially in the lumbar and sacral regions. The vertebrae are
generally without epiphyses to their centra. The cervical vertebrae in
living forms have saddle-shaped articulating surfaces, and many of
them bear ribs. The thoracic ribs in almost all birds have large
uncinate processes. The sternum is very large, and the ribs are always
attached to its sides, not as in many reptiles to anybackwardly
projecting process or processes. The sternum ossifies from two or more
centres.
The skull is extremely light, and its component bones show a great
tendency to fuse together completely. The facial part of the skull is
prolonged into a beak, chiefly formed of the premaxillae; this beak is
in all modern birds devoid of teeth, and is coated externally with a
horny epidermal sheath. The quadrate is large and freely movable. The
supratemporal arcade[1] is imperfect, while the infratemporal
arcade[95] is complete. There are no postorbital or postfrontal bones.
Neither parotic processes nor an interparietal foramen occur. There
are commonly large pre-orbital vacuities. The palatines and pterygoids
never form a secondary bony palate as in Crocodiles. Part of the floor
of the skull is formed by a wide _basitemporal_ (paired in the embryo)
which is continued in front as a long slender _rostrum_; these
structures have replaced the parasphenoid of Ichthyopsids. Cartilage
or bone is always developed in the sclerotic. The first branchial arch
is well developed, the hyoid arch but slightly. The coracoids are
large, and the clavicles are nearly always united forming the
_furcula_. There is no separate interclavicle and hardly any trace of
a precoracoid.
The anterior limbs form wings, and the manus is in the adult always
much modified, never having more than three digits. The three bones of
the pelvis are, except in Archaeornithes, always ankylosed together in
the adult, and the ilium is greatly prolonged in front of the
acetabulum, which is perforated. The ilia are not connected with the
sacrum by ossified sacral ribs. The pubes and ischia are directed
backwards parallel to one another, and except in a very few forms
never meet their fellows in ventral symphyses. The fibula is generally
much reduced. The proximal tarsal bones are always ankylosed to the
tibia, and the distal tarsals to the metatarsals, so that the ankle
joint is _intertarsal_. The first metatarsal is nearly always free.
The pes never has more than four digits in the adult.
The class Aves is most conveniently divided into two subclasses: 1.
Archaeornithes. 2. Neornithes.
Subclass I. ARCHAEORNITHES.
The only form referred to this subclass of extinct birds is
_Archaeopteryx_[96], the earliest known bird. In this animal the
skeleton does not seem to be pneumatic. The cervical and trunk
vertebrae are generally thought to be flat, certainly their
articulating surfaces are not saddle-shaped. There is no long compound
sacrum as in modern birds. The tail is longer than the whole body, the
caudal vertebrae are twenty in number, they gradually taper as traced
away from the trunk, and each bears a pair of feathers. The posterior
caudal vertebrae are not united together to form a _pygostyle_. The
upper jaw bears thirteen pairs of conical teeth, planted in distinct
sockets in the maxillae and premaxillae, but the mandible has only three
pairs. The presence of these teeth forms the most essential difference
between the skull of _Archaeopteryx_ and that of modern birds, and the
fact that they occur on the premaxillae renders it improbable that a
horny beak was present. There is a ring of ossifications in the
sclerotic. The ribs do not show uncinate processes, and articulate
with the vertebrae by single heads not divided into capitula and
tubercula. Abdominal ribs appear to have been present. The furcula is
large, and the scapula has a well developed acromion. The sternum is
unknown. The radius and ulna are approximately equal in size. In the
manus the first, second and third digits[97] are present, each
terminated by a claw. The second digit is considerably the longest,
while the third includes four phalanges. The three bones of the pelvis
probably remained distinct throughout life. The tarsals are ankylosed
respectively to the tibia and metatarsals as in other birds. The
metatarsals are ankylosed together, and the pes has four digits.
_Subclass II._ NEORNITHES.
To this subclass may be referred all known birds except
_Archaeopteryx_. They all agree in having a short tail whose component
vertebrae are commonly ankylosed together forming a pygostyle. The
three metacarpals do not all remain distinct. The bones of the pelvis
are ankylosed together, and to a large though variable number of
vertebrae. There are three orders, the Ratitae, Odontolcae, and
Carinatae.
_Order_ 1. RATITAE.
The Ratitae differ from _Archaeopteryx_ and the great majority of
Carinatae in being flightless. The bones are generally not pneumatic,
containing marrow instead of air, in the Ostrich however they are very
pneumatic. The tail is short and the posterior caudal vertebrae are
generally ankylosed together forming a pygostyle. The pectoral girdle
has comparatively a much smaller size than in Carinatae, clavicles are
small or absent, and the scapula and coracoid lie nearly in the same
straight line. The ilium and ischium do not as in Carinatae unite
posteriorly, and enclose a foramen except in very old Rheas and Emeus.
The quadrate articulates with the cranium by a single head. The vomers
unite and form a broad plate, separating the palatines, pterygoids and
basisphenoidal rostrum.
The anterior limbs are greatly reduced in size or even absent, while
the posterior limbs are greatly developed and adapted for running. The
tibia and fibula are quite distinct.
Many ornithologists agree that the various forms grouped together as
Ratitae are not all very closely allied to one another, that they
resemble one another mainly in having lost the power of flight, and do
not form a natural group.
The Ratitae include the following groups:--
_Æpyornithes_[98], huge extinct birds from Madagascar.
_Apteryges_, including the Apteryx of New Zealand.
_Dinornithes_[99], the Moas, huge extinct birds from New Zealand, and
some of the neighbouring islands.
_Megistanes_, including the Cassowaries (_Casuarius_) of Australia,
New Guinea, and some of the neighbouring islands; and the Emeus
(_Dromaeus_) of Australia.
_Rheornithes_, including the Rheas of S. America.
_Struthiornithes_, including the Ostriches (_Struthio_) now living in
Africa, and found fossil in N. India and Samos.
_Order_ 2. ODONTOLCAE.
This order includes only an extinct N. American bird
_Hesperornis_[100]. The jaws are provided with a series of sharp teeth
placed in continuous grooves, but the premaxillae are toothless, and
were probably sheathed in a horny beak. The rami of the mandible are
not ankylosed together in front. The skeleton is not pneumatic. The
cervical vertebrae have saddle-shaped articulating surfaces as in
ordinary birds, and the thoracic vertebrae are not ankylosed together.
The tail is comparatively long, and formed of twelve vertebrae with
only slight indications of a pygostyle. The ribs have uncinate
processes. The anterior limb is quite vestigial, being reduced to a
slender humerus. The posterior limb is very powerful and adapted for
swimming.
_Order_ 3. CARINATAE.
This order includes the vast majority of living birds. The cervical
vertebrae have saddle-shaped articulating surfaces (except in the
Ichthyornithiformes). The posterior caudal vertebrae are ankylosed
forming a pygostyle. The quadrate articulates with the cranium by a
double head. In all except the Tinamidae the vomers are narrow behind
and not interposed between the palatines, pterygoids and
basisphenoidal rostrum. The sternum has a median keel, and the
anterior limbs are in the great majority of cases adapted for flight.
Clavicles are well developed, and the scapula and coracoid are nearly
at right angles to one another. The various groups into which the
Carinatae are divisible are shown in the table on pp. 40-42. Their
special characters will not be dealt with.
[Illustration FIG. 55. _Gallus bankiva_ var. _domesticus_. THE LEFT
HALF OF THE SKELETON. The skull, vertebral column, and sternum are
bisected in the median plane. (After Marshall and Hurst.)
A, acetabulum. B, cerebral fossa. CB, cerebellar fossa. CL, clavicle.
CO, coracoid. CR, cervical rib. C 1 = one, first cervical vertebra.
FE, femur. HC, ventral end of clavicle. HU, humerus. HY, hyoid. IF,
ilio-sciatic foramen. IL, ilium. IS, ischium. L, lachrymal. MC 3,
postaxial metacarpal. MN, mandible. MS, xiphoid processes. MT,
tarso-metatarsus. MT 1, first metatarsal. N, nasal. OP, optic foramen.
P, premaxillae. PB, pubis. PL, palatine. PY, pygostyle. R, radius. RC,
radial carpal. S, keel of sternum. SC, scapula. T, tibio-tarsus. TH 4,
fourth thoracic vertebra. U, ulna. UC, ulnar carpal. UP, uncinate
process. Z, infra-orbital bar. 1, 2, 3, 4, first, second, third and
fourth digits of pes. 3, pre-axial, 4, middle, and 5, postaxial digit
of manus.]
FOOTNOTES:
[94] M. Fürbringer, _Untersuchungen zur Morphologie und Systematik der
Vögel_, I. and II. Amsterdam, 1888. Cf. H. Gadow, _Nature_, XXXIX.
1888, pp. 150 and 177.
T.H. Huxley, "On the classification of birds." _P.Z.S._, London, 1867.
E. Selenka and H. Gadow, _Vögel_ in Bronn's _Classen und Ordnungen des
Thierreichs_ 1869-1890.
[95] See p. 283.
[96] R. Owen, _Phil. Trans._, vol. CLIII., p. 33; 1863. T.H. Huxley,
_P. R.S._, vol. XVI., p. 243; 1868. C. Vogt, _Rev. Scient._, ser. 2,
tom. 9, p. 241; 1879. C.H. Hurst, _Nat. Sci._, vol. III., p. 275;
1893; vol. VI., pp. 112, 180, 244; 1895. W.P. Pycraft, _Nat. Sci._,
vol. V., pp. 350 and 437; 1894; and vol. VIII., p. 261; 1896.
[97] According to Hurst the fourth and fifth digits are also present.
[98] See C.W. Andrews, _P.Z.S._, 1894, p. 108.
[99] See T.J. Parker, _Tr. Zool. Soc. London_, vol. XIII., pt. 2,
1895, and F.W. Hutton, several papers in _Tr. N. Zealand Inst._, 1893
and 1895.
[100] See O.C. Marsh. _Odontornithes. A monograph of the extinct
toothed birds of N. America._ New Haven, 1880.