GYMNOPHIONA. The skull bears a considerable resemblance to that of Labyrinthodonts, especially in the arrangement of the bones which bound the mouth cavity. The cranium is very hard, and is covered by a complete bony roof formed mainly of the exoccipitals, parietals, frontals, prefrontals, nasals and premaxillae. The nasals and premaxillae are sometimes ossified continuously. There is a median unpaired ethmoid whose dorsal end appears at the surface wedged in between the frontals and parietals. The bone generally regarded as the squamosal[63] is very large, and it and the maxillae generally together surround the orbit, which, in _Epicrium_, has in it a ring of bones. The palatines form long tooth-bearing bones fused with the inner sides of the maxillae; they nearly surround the posterior nares. The quadrate bears the knob, and the angular the cup for the articulation of the mandible,--a very primitive feature. The mandible is also noticeable for the enormous backward projection of the angular. * * * * * ANURA. In Anura the skull is very short and wide owing to the transverse position of the suspensorium. There is often a small ossification representing the quadrate. Sometimes as in _Hyla_ and _Alytes_ there is a fronto-parietal fontanelle. As compared with the skull in Urodela the chief characteristics of the skull of Anura are:-- 1. the presence of a sphenethmoid, 2. the union of the frontals and parietals on each side, 3. the occasional occurrence of small supra- and basi-occipitals, 4. the backward growth of the maxillae and its connection with the suspensorium by means of the quadratojugal, 5. the dagger-like shape of the parasphenoid, 6. the occurrence of a definite tympanic cavity, 7. the frequent occurrence of a predentary or mento-meckelian ossification in the mandible. The skull of _Pipa_ is abnormal, being greatly flattened and containing little cartilage. The fronto-parietals are fused, and there is no sphenethmoid. The quadrates are well developed and the squamosals and parasphenoid differ much from those of other Anura. HYOID AND BRANCHIAL ARCHES. In larval Amphibia the hyoid and four branchial arches are generally present, and in adult Ichthyoidea they are frequently almost as well represented as in the larva, and are of use in strengthening the swallowing apparatus. They are very well seen in _Siredon_, and consist of a hyoid attached by ligaments to the suspensorium, followed by four branchial arches of which the first and second are united by a copula (fig. 29, D, 8), while the third and fourth are not. The hyoid is not always the largest and best preserved of the arches, for sometimes as in _Spelerpes_ one of the branchials is far larger than the hyoid. Four branchial arches occur in _Siren_ as in _Siredon_, but in _Proteus_ there are only three.