In the Proteidea there are three or four branchial arches,
each usually consisting of two cartilaginous, or ossified, pieces
on each side. In the
Salamandridea, there are, primitively,
four branchial arches, but of these, portions of only the two
anterior remain in the adult. Four are developed in the
Caecilia, and three of these are permanent.
Some peculiarities exhibited by the skulls of the Gymnophiona,
and by the
Labyrinthodonta, are worthy of especial
notice.
In the former, e. g., in
Ichthyophis glutinosa, the skull is
covered by a complete bony roof, formed, mainly, by the exoocipitals,
parietals, frontals, prefrontals, nasals, and ascending
processes of the premaxillaries. Between the ex-occipitals,
the parietal, and the frontal, above, the maxilla, in front, and
the quadrate, behind and below, lies a bone which appears to
answer to the bone (
z) of the Frog, and to its quadrato-jugal.
Between the nostril and the maxilla, the nasal bone and the
premaxi la, there is a bone which seems to be an ossification
of the cartilaginous
ala nasi.
Another bone nearly encircles
the orbit, and, as a supra - and postorbital bone, has no analogue
among existing
Amphibia.
 |
Fig. 56. - Side and upper views of the skull of Trematosaurus. The sculpture of the cranial
bones is not represented in the lower half of the upper view of the skull, in order to
show the sutures more distinctly. |
The palatine bones surround
the posterior and outer margins of the posterior nares,
and then extend back on the inner side of the maxilla, in a
manner unlike any thing observed among other existing
Amphibia.
But in the
Labyrinthodonta, both this disposition
of the palatine and the complete roofing over of the skull by
bone are repeated, and there is a postorbital bone.
The Labyrinthodont skull is further characterized by the
development of distinct pointed epiotics, like those of fishes,
and of paired ossifications, which take the place of the supraoccipital,
as in many
Ganoidei. In many Labyrinthodonts
the articular element of the lower jaw is completely ossified.
Archegosaurus possessed branchial arches when young,
and there can be little doubt that the other Labyrinthodonts
resembled it in this respect.
The limbs and their arches are completely absent in the
Gymnophiona, and, apparently, in the extinct
Ophiderpeton of the Carboniferous formation. In all other
Amphibia the
pectoral arch and limbs are present, and, in all but
Siren, the
pelvic arch and limbs. The anterior and posterior limb-arches
consist of a continuous cartilage on each side, divided by an
articular surface into a smaller dorsal moiety, and a more expanded
ventral portion. The dorsal moieties are, respectively,
the scapula and the ilium. The ventral moieties are divided
by notches, or fontanelles, into two portions - an anterior, precoracoidal,
or pubic part, and a posterior, coracoidal, or ischial
part.