The very remarkable extinct
reptiles which constitute this group, present a large series of
modifications intermediate in structure between existing Reptilia
This transitional character of the Ornithoscelidan skeleton
is most marked in the pelvis and hind-limbs.
If the pelvis of any existing reptile be compared with that
of any existing bird, the following points of difference will be
1. In the reptile (Fig. 78, C), the ilium is not prolonged
in front of the acetabulum; and the acetabulum is either
wholly closed by bone, or presents only a moderate-sized
fontanelle, as in the Crocodilia.
In the bird (Fig. 78, A.), the ilium is greatly prolonged iu
front of the acetabulum, and the roof of the acetabular cavity
is a wide arch, the inner wall of that cavity remaining membranous.
The anterior pier of the arch, or prae-acetabular process,
extends farther downward than the posterior pier, or
But, in all the Ornithoscelida
, the ilium extends far in
front of the acetabulum, and furnishes only a widely-arched roof
to that cavity, as in birds. It retains a reptilian character in
the further proportional extension of the post-acetabular process
downward (Fig. 78, B.).
2. The ischium, in the reptile (Fig. 78, C), is a moderately
elongated bone, which becomes connected with the pubis in
the acetabulum, and extends downward, inward, and somewhat
backward, to unite with its fellow in a median ventral symphysis.
The obturator space is not interrupted by any forward
process of the outer and anterior half of the ischium.
In all birds (Fig. 78, A.), the ischium is elongated and inclined
backward, the backward direction being least marked
and most in Rhea
. The ischia never come together
directly in a median ventral symphysis, though they
unite dorsally in Rhea
. The anterior edge of the external, or
acetabular, half of the ischium very generally sends off a process
which unites with the pubis, thus dividing the obturator
In all the Ornithoscelida
(Fig. 78, B.), in which I have
been able to identify the bone (Thecodontosaurus, Teratesaurus,
Megalosaurus, Iguanodon, Stenopelyx, Hadrosaurus,
), the ischium is greatly elongated. In Iguanadon it has the obturator
process characteristic of the same
bone in Birds; and I imagine that the same process is seen in Compsognathus
. In Hypsilophodon
there can be no mistake
about the matter, and the remarkable slenderness and prolongation
of the ischium give it a wonderfully ornithic character.
the slenderness and prolongation are even carried
beyond what are to be seen in Birds. I am disposed to
think, however, that, as was certainly the case in Hypsilophodon
the ischia united in a median ventral symphysis in all the Ornithoscelida
3. In all reptiles the pubis is inclined forward, as well as
downward, toward the ventral median line. In all, except the
Crocodile, it takes a considerable share in the formation of the
acetabulum; and the ossified pubis unites directly with its
fellow in the middle line.
The pubes of Compsognathus
are, unfortunately, obscured
by the femora. They seem to have been very slender; and to
have been directed forward and downward, like those of Lizards.
Some lizards, in fact, have pubes which, if the animal were
fossilized in the same position as Compsognathus
, would be very
similar in form and direction. Hypsilophodon
, however, affords
unequivocal evidences of a further step toward the bird. The
pubes are not only as slender and elongated as in the most
typical bird, but they are directed downward and backward
parallel with the ischia, thus leaving only a very narrow and
elongated obturator foramen, which is divided by the obturator
It remains to be seen how far the hypsilophodont modification
extended among the Ornithoscelida
. The remains of Compsognathus
and of Stenopelyx
tend to show that it was
by no means universal.
As to the hind-limb, in existing reptiles-
- The proximal end of the tibia has but a very small, or
quite rudimentary, cnemial crest, and it presents no ridge for
the fibula on its outer side.
- The flattened sides of the distal end of the tibia look,
the one directly forward, or forward and inward; and the
other backward, or backward and outward. And when the
posterior edges of the two condyles of the proximal end of the
tibia rest on a flat surface which looks forward, the long axis
of the distal end is either nearly parallel with that surface, or
is inclined obliquely from in front and without, backward and
- There is no depression on the anterior face of the tibia
for the reception of an ascending process of the astragalus.
- The distal end of the fibula is as large as, or larger than,
the proximal end, and articulates largely with a facet on the
outer part of the astragalus.
- The astragalus is not depressed and flattened from above
downward, nor does it send a process upward in front of the tibia.
- The astragalus remains quite free from the tibia.
In all these respects, the leg of any existing bird (see
Fig. 78) is very strikingly contrasted with that of the reptile:
- The proximal end of the tibia is produced forward and
outward into an enormous cnemial crest, in all walking and
swimming birds (Fig. 78, A.); and, on the outer side, there
is a strong ridge for the fibula.
- When the posterior edges of the condyles of the tibia
rest upon a flat surface, the one flat face of the distal end of
the bone looks outward as well as forward, and the other inward
as well as backward. Further, the long axis of the distal
end is inclined, at an angle of 45° to the flat surface, from
within and in front, backward and outward, thus exactly reversing
the direction in the reptile.
- There is a deep longitudinal depression on the anterior
face of the distal end of the tibia, which receives an ascending
process of the astragalus.
- The distal end of the fibula is a mere style, and does not
articulate with the astragalus.
- The astragalus is a much-depressed bone, with a concave
proximal, and a convex, pulley-like, distal, surface. A process
ascends from its front margin in the groove on the front face
of the tibia. This process is comparatively short, and perforated
by two canals for the tibialis anticus and extensor communis,
in the Fowl; while in the Ostrich and Emeu it is extremely
long and not so perforated.
- The astragalus becomes anchylosed with the tibia
(though it remains distinct for a long time in the Ostrich and Rhea, and in some breeds of fowls).
In the Ornithoscelida
- There is a great cnemial crest and a ridge for the fibula.
- The disposition of the distal end of the tibia is literally
that observed in the Bird.
- There is a fossa for the reception of the ascending process
of the astragalus.
- The distal end of the fibula is much smaller than the
pioximal, though not so slender as in Aves.
- The astragalus is altogether similar to that of a bird,
with a short ascending process.
- The astragalus appears to have remained distinct from
the tibia throughout life in Iguanodon, Megalosaurus, and
many other genera; but it seems to have become anchylosed
in Compsognathus, Ornithotarsus, and Euskelosaurus.
The reptiles belonging to this group are for the most part
of very large size, and some of them, as the Iguanodon
among the largest of known terrestrial animals. They occur
throughout the whole range of the Mesozoic formations,
being represented by Thecodontosaurus, Palaeosaurus, Teratosaurus,
and other genera in the Trias; by Scelidosaurus
in the Lias; by Megalosaurus, Poikilopleuron,
Euskelosaurus, Hylaeosaurus, Polacanthus, Acanthopholis,
Iguanodon, Hadrosaurus, Trachodon,
middle and upper Mesozoic strata.
There is no evidence that Megalosaurus
, or Iguanodon
possessed any dermal armor; but several genera (e. g., Scelidosaurus,
) had osseous
dermal scutes, sometimes produced into prodigious spines.
The faces of the centra of the vertebrae are slightly amphicoelous,
or nearly flat; but those of the anterior dorsal and
cervical regions seem, in some cases, to have been opisthocoelous.
The sacrum seems to have consisted of at fewest four
vertebrae, which in some (Scelidosaurus
) are crocodilian, in
) take on a somewhat ornithic character.
The caudal region had many and long vertebrae, between
which the chevron-bones are attached. The rami of the chevron-bones have their vertebral ends united by bone.
The thoracic vertebral ribs are very strong; but the sternal
ribs and sternum are unknown. However, there is some reason
to think that the sternum was broad and expanded. Abdominal
dermal ribs are developed in some species, if not in all.
The structure of the skull seems to have been intermediate,
in many respects, between the crocodilian and the lacertilian
types. In Iguanodon
, the extremities of
the premaxillae appear to have been edentulous and beak-like;
and the symphysis of the mandible is excavated to receive the
beak, almost as in the mandible of a Parrot.
The teeth vary extremely, from the sharp, recurved, serrated
fangs of Megalosaurus
, to the broad grinders, wearing
down by mutual attrition, of Iguanodon
. Their mode of implantation
varies, but they are not anchylosed to the jaws.
The scapula is vertically elongated, narrow, and devoid of
any acromial process; the coracoid rounded and without fontanellos
No Ornithosoelidan is known to have possessed a clavicle.
The fore-limb is shorter, and often much shorter, than
the hind-limb. The structure of the manus is not certainly
The femur usually has a strong inner trochanter; and its
distal end is particularly bird-like, in the development of a
strong ridge, which plays between the tibia and the fibula.
The metatarsals are elongated, and fit together in such a
way that they can hardly, if at all, move on one another. The
inner and outer digits are either shorter than the rest, or quite
rudimentary; and the third digit is the longest, as in birds in
are divisible into two sub-orders, the Dinosauria
and the Compsognatha
. The type of the latter
division is the wonderful little extinct reptile, Compsognathus
which differs from the Dinosauria
in the great length
of the centra of the cervical vertebrae, and in the femur being
shorter than the tibia. It has a light bird-like head (provided
with numerous teeth), a very long neck, small anterior limbs,
and very long posterior limbs. The astragalus appears to
have been anchylosed with the tibia, as in birds. A single
specimen only of this reptile has been obtained, in the Solen