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:--