The skull of Siren resembles that of Menobranchus in several respects, as in the forward direction of the suspensorium and in the absence of maxillae, but differs in the possession of nasals, in the toothless condition of the premaxillae and dentaries, and in the fusion and dentigerous condition of the vomers and palatines.
Amphiuma has a skull which, though narrow and elongated, differs from those of Menobranchus, Proteus, and Siren, and resembles those of higher types in the following respects:—
(1) the suspensorium projects nearly at right angles to the cranium instead of being directed forwards, (2) the maxillae are well developed, and the premaxillae are completely ankylosed together, (3) there are no palatines.
The skulls of Megalobatrachus, Cryptobranchus and Siredon resemble those of the highest Urodeles the Salamanders in their wide form, in having the pro-otics distinct from the exoccipitals which are ossified continuously with the epi-otics and opisthotics, and in having no palatines, but differ in having the two premaxillae separate, and in the arrangement of the vomerine teeth which in Megalobatrachus and Cryptobranchus are placed along the anterior boundaries of the bones, these meeting in the middle line. In Siredon the vomers are separated by the very large parasphenoid.
The suspensorium in Megalobatrachus and Cryptobranchus projects at right angles to the cranium; in Siredon it projects somewhat downwards and forwards as in the Salamandrina.
Modifications of the vomers, pterygoids and palatines accompany the changes of the larval ichthyoid Siredon into the adult salamandroid Amblystoma, the vomers especially come to resemble to a much greater extent those of the Salamandrina.
The ossification of the skull in the Salamandrina is carried further than in the Ichthyoidea, though the supra-occipital and basi-occipital are not ossified. The skull differs from that in the Ichthyoidea in the size of the part of the vomero-palatines which lies in front of the teeth, in the frequent union of the two premaxillae and in the ossification of all the periotic bones continuously with the exoccipital.
The skull differs from that of Anura in the following respects:—
(1) the bones of the upper jaw do not form a complete arch standing away from the cranium, and the maxillae are not united to the quadrates by quadratojugals, (2) the long axis of the suspensorium passes obliquely downwards and forwards instead of downwards and backwards, (3) there is no sphenethmoid encircling the anterior end of the brain, its place being partly taken by a pair of orbitosphenoids, (4) there is no definite tympanic cavity.
