In some forms such as Bombinator, Pipa, Discoglossus and Alytes they are opisthocoelous; in others like Pelobates they are variable.
The Skull[62].
Cranium and mandible.
In the Amphibian skull there are as a rule far fewer bones than in the skull of bony fish. The primordial cartilaginous cranium often persists to a great extent. Only quite a few ossifications take place in it; namely in the occipital region—the exoccipitals, further forwards—the pro-otics, and at the boundary of the orbital and ethmoidal regions—the sphenethmoid. The basi-occipital and basisphenoid are never ossified. As in Mammalia there are two occipital condyles formed by the exoccipitals.
Large vacuities commonly occur in the cartilage of both floor and roof of the primordial cranium. These are roofed over to a greater or less extent by the development of membrane bone. Thus on the roof of the cranium there are paired parietals, frontals, and nasals, and on its floor are paired vomers, and a median unpaired parasphenoid.
In all living forms the parietals meet and there is no interparietal foramen, though this exists in Labyrinthodonts.
The palato-pterygo-quadrate bar is united at each end with the cranium, but elsewhere in most cases forms a wide arch standing away from it. The suspensorium is, as in Dipnoi and Holocephali, autostylic. The palato-pterygo-quadrate bar sometimes remains entirely cartilaginous, sometimes its posterior half is ossified forming the quadrate. In connection with it a number of membrane bones are generally developed, viz. the maxillae, premaxillae, palatines, pterygoids, quadratojugals, and squamosals. The pterygoids are, however, sometimes partially formed by the ossification of cartilage. The cartilage of the lower jaw and its investing membrane bones generally have much the same relations as in bony fishes.
Urodela. The skulls of the various Urodeles show an interesting series of modifications and differ much from one another, but all agree in the absence of the quadratojugals, in the fact that the palatines lie parallel to the axis of the cranium, and in the large size of the parasphenoid.
The lower types Menobranchus, Siren, Proteus, and Amphiuma have longer and narrower skulls than do the higher types.
Menobranchus has a very low type of skull which remains throughout life in much the same condition as that of a young tadpole or larval salamander. The roof and floor of the cranium internal to the membrane bones are formed of fibrous tissue, not of well-developed cartilage. The epi-otic regions of the skull are ossified, forming a pair of large bones which lie external to, and distinct from, the exoccipitals. Proteus and the Labyrinthodonts are the only other Amphibia which have these elements separately ossified. The parietals send a pair of long processes forwards along the sides of the frontals. Nasals and maxillae are absent, as is likewise the case in Proteus. Teeth are borne on the vomers, premaxillae, pterygoids, dentaries and angulo-splenials. The suspensorium is forwardly directed.