The Class Amphibia

The only clearly diagnostic characters of this class as compared with Fishes are the following:
1. Amphibia have no fin-rays.
2. When limbs are present they contain the same skeletal elements as those of the higher Vertebrata.

Certain other structural peculiarities are common to the whole of the Amphibia, and are very characteristic of them without being diagnostic. Thus:

1. The body is usually devoid of any exoskeleton, and when scales, or scutes, are present in recent Amphibia, they are concealed within the skin (Caecilia, Ephippifer). In the extinct Labyrinthodonta, the dermal armor is confined to the ventral region of the body.
   
2. The vertebral centra are always represented by bone.
   
3. The sacrum rarely consists of more than one vertebra, though there are individual exceptions to this rule, as in Menopoma.
   
4. The suspensorial apparatus of the mandible is continuous with the skull, which has two occipital condyles, and no completely ossified basi-occipital.
   
5. There are no sternal ribs.

The Amphibia are divisible into the following groups:

1. A distinct and often long tail; the vertebrae amphicoelous or opis. thocoelous; the proximal elements of the tarsus not elongated.
1. Two or four limbs; no scutes or scales.
I. Saurobatrachia or Urodela.
1. External branchiae or gill-clefts persistent, or disappearing only in advanced age; no eyelids; vertebrae amphicaelous; carpus and tarsus cartilaginous.
1. Proteidea.
2. No branchiae or branchial clefts in the adult; eyelids present; carpus and tarsus more or less ossified; vertebrae commonly opisthocoelous.
2. Salamandridea,
2. Limbs absent, or all four present. Three large pectoral osseous plates and an armor of small scutes on the ventral surface of the body; vertebrae amphicoelous; walls of the teeth more or less folded.
   
   II. Labyrinthodonta..
2. Tail obsolete in the adult.

1. Limbs absent; numerous minute dermal scutes imbedded in the integument of the serpentiform body.
   
   III. Gymnophiona.
2. All four limbs present, and the proximal elements of the tarsus much elongated; the body short, and the integument devoid of small scutes, though dermal osseous plates are sometimes developed in it.
   
   IV. Batrachia or Anura.

The integument in most Amphibia is soft and moist, as in the Frog, where numerous glands open upon its surface. The Gymnophiona are exceptional, among existing Amphibia, in possessing small, rounded, flexible scales, like the cycloid scales of fishes, imbedded within the wrinkled integument.

In certain Batrachia (Ceratophrys dorsata, Ephippifer aurantiacus), flat dermal bony plates are developed in the dorsal integument, and become united with some of the subjacent vertebrae. Many of the extinct Labyrinthodonta, and probably the whole of the members of that group, possessed an exoskeleton which appears to have been confined to the ventral surface of the body. Under the anterior part of the thorax there is a sort of plastron composed of one median and two lateral plates. The median plate is rhomboidal. The lateral ones are somewhat triangular, and unite with the anterolateral margins of the median plate by one side, sending a process upward and backward from their outer angles. The outer surfaces of all these plates exhibit a sculpture, which radiates from the centre of the median plate and from the outer angles of the lateral plates. These plates are in close relation with the pectoral arch, and probably represent the interclavicle and clavicles.

Minute bony plates cover the surface of the throat in a small African Labyrinthodont, Micropholis. I have not met with dermal ossicles in this position in other Labyrinthodonts. But in Archegosaurus, Pholidogaster, Urocordylus, Keraterpeton, Ophiderpeton, Ichthyerpeton, the integument between the thoracic plates and the pelvis presents regularly-disposed rows of small elongated ossicles, which, for the most part, converge from without, forward and inward, toward the middle line. No trace of these appears upon the tail, nor in any part of the dorsal region of the body, nor on the limbs.

The endoskeleton of the Amphibia is least complete in Archegosaurus, where the centra of the vertebrae are represented only by bony rings, the ribs and the neural arches being well ossified. In other Labyrinthodonts of the same (Carboniferous) epoch, however, such as Anthracosaurus, the centra of the vertebrae are completely ossified biconcave disks, very like the centra of the vertebrae of Ichthyosaurus.