Amphibia differ markedly from Pisces in the fact that in the more abundant and familiar forms the skin is naked, and that when the integument is prolonged into median fins they are devoid of fin-rays. The notochord may persist, but bony vertebral centra are always developed. These are sometimes biconcave, sometimes procoelous, sometimes opisthocoelous. There is only one sacral vertebra, except in rare cases. The cartilaginous cranium persists to a considerable extent but is more or less replaced by cartilage bone, and overlain by membrane bone. The basi-occipital is not completely ossified, and the skull articulates with the vertebral column by means of two occipital condyles formed by the exoccipitals.

There is a large parasphenoid, but there are no ossifications in the basisphenoidal, presphenoidal, and alisphenoidal regions. In most cases the epi-otics and opisthotics are ossified continuously with the exoccipitals.

The palato-pterygo-quadrate bar is firmly united to the cranium, so the skull is autostylic. The palatines and pterygoids are membrane bones. Teeth are nearly always borne on the vomers and commonly on the maxillae and premaxillae. There are no sternal ribs, and the sternum is very intimately related to the pectoral girdle. There are no obturator foramina. The limbs are as in the higher vertebrata, divisible into upper arm, fore-arm, and manus (wrist and hand), and into thigh, shin, and pes (ankle and foot) respectively. The posterior limb is, as a rule, pentedactylate, but in nearly every case the pollex is vestigial or absent.

Order 1. Urodela[51].

The Urodela are elongated animals with a naked skin, a persistent tail, and generally four short limbs.

The vertebral centra are opisthocoelous or biconcave, and there are numerous precaudal vertebrae. Portions of the notochord commonly persist in the intervertebral spaces. In the skull there is no sphenethmoid forming a ring encircling the anterior end of the brain, its place being in many cases partly taken by a pair of orbitosphenoids. There is no quadratojugal, and the quadrate is more or less ossified. The mandible has a distinct splenial, and the articular is ossified.

There is no definite tympanic cavity. The hyoid apparatus is throughout life connected to the quadrate by ligament, and a large basilingual plate does not occur. The ribs are short structures with bifurcated proximal ends. In the pelvis the pubis remains cartilaginous, and there is a bifid cartilaginous epipubis. The bones of the fore-arm and shin remain distinct, and the manus never has more than four digits.

Suborder (1). Ichthyoidea.

The vertebrae are amphicoelous, but the notochord remains but little constricted throughout the whole length of the vertebral column. Three or four branchial arches nearly always persist in the adult. The cartilages of the carpus and tarsus remain unossified.

The Ichthyoidea may be subdivided again into two groups:—

A. Perennibranchiata, whose chief distinguishing skeletal characters are that the skull is elongated, the premaxillae are not ankylosed, the maxillae are vestigial or absent; there are sometimes no nasals, and the palatines bear teeth;

e.g. Siren, Proteus, Menobranchus.

B. Derotremata, whose chief distinguishing skeletal characters are that there are large maxillae and nasals; teeth are borne by both maxillae and premaxillae; there are no palatines; and both pectoral and pelvic limbs are always present;

e.g. Amphiuma, Megalobatrachus, Cryptobranchus.

Suborder (2). Salamandrina.

The vertebrae are opisthocoelous. The skull is broad, and teeth are borne by both premaxillae and dentaries. Nasal bones are present. The remains of only two branchial arches are found in the adult. The carpus and tarsus are more or less ossified.

This suborder includes the Newts (Molge), Salamanders (Salamandra), and Amblystoma.

Order 2. Labyrinthodontia[52].

These are extinct Amphibia with a greatly developed dermal exoskeleton, which is generally limited to the ventral surface. The body and tail are long and in some cases limbs are absent. The teeth are pointed and often have the dentine remarkably folded. The vertebrae are amphicoelous, and are generally well ossified. The skull is very solid, and has a greatly-developed secondary roof which hides the true cranium and is very little broken up by fossae. Paired dermal supra-occipitals are found, and there is an interparietal foramen. The epi-otics and opisthotics form a pair of bones distinct from the exoccipitals. Four simple limbs of moderate length are generally present, and in some cases all four limbs are pentedactylate. Among the better known genera of Labyrinthodonts are Mastodonsaurus, Nyrania, and Archegosaurus.

Order 3. Gymnophiona[53].

These animals form a group of abnormal worm-like Amphibia having an exoskeleton in the form of subcutaneous scales arranged in rings. The vertebrae are biconcave and are very numerous; very few however belong to the tail. The skull has a complete secondary bony roof, the mandible bears teeth and has an enormous backward projection of the angular. The hyoid arch has very slender cornua and no distinct body, it is attached neither to the cranium nor to the suspensorium. The ribs are very long and there are no limbs or limb girdles.

Order 4. Anura.

These are tailless Amphibia, which except in a few instances, are devoid of an exoskeleton. The vertebrae are as a rule procoelous, and are very few in number. The post-sacral part of the spinal column ossifies continuously, forming an unsegmented cylindrical rod, the urostyle. Remains of the notochord persist, lying vertebrally, i.e. enclosed within the centra of the several vertebrae, and not as in Urodela lying between one vertebra and the next. The skull is very short and wide. The mandible is almost always, if not invariably, toothless.

The frontals and parietals on each side are united so as to form a pair of fronto-parietals, and a girdle-like sphenethmoid is present.

The quadrate is not generally ossified. A predentary or mento-meckelian bone is commonly present in the mandible, and a single bone represents the angular and splenial. The branchial arches are much reduced in the adult, and the distal ends of the cornua unite to form a flat basilingual plate of a comparatively large size.

Ribs are very little developed. Clavicles are present. The ilia are very greatly elongated. The anterior limb has four well-developed digits and a vestigial pollex, and is of moderate length; the radius and ulna have fused. The posterior limb is greatly elongated and is pentedactylate; the tibia and fibula are fused, while the calcaneum and astragalus are greatly elongated, and it is largely owing to them that the length of the limb is so great. The group includes the Frogs and Toads, the predominant Amphibia of the present time.