. The biology of the amphibia. Amphibians. 460 THE BIOLOGY OF THE AMPHIBIA iferous through the Permian. The centra were double, that is, the intercentrum and pleurocentrum formed complete rings and the neural arch attached to one or both of them. The occipital condyle was single or triple, and a well-ossified basioccipital and basisphenoid were present. The palate was well ossified, broad pterygoid bones leaving very small vacuities between them and the parasphenoid. The various genera in the suborder have been grouped into the families Anthracosauridae, Loxommidae, Pho- lidogasteridae, and Cr
. The biology of the amphibia. Amphibians. 460 THE BIOLOGY OF THE AMPHIBIA iferous through the Permian. The centra were double, that is, the intercentrum and pleurocentrum formed complete rings and the neural arch attached to one or both of them. The occipital condyle was single or triple, and a well-ossified basioccipital and basisphenoid were present. The palate was well ossified, broad pterygoid bones leaving very small vacuities between them and the parasphenoid. The various genera in the suborder have been grouped into the families Anthracosauridae, Loxommidae, Pho- lidogasteridae, and Cricotidae. SUBORDER 2. RACHITOML—The rachitomous labyrintho- donts lived during the Permian and Triassic periods. They differ from their embolomerous ancestors in that each vertebra consists of a half-moon-shaped intercentrum and one or two pairs of pleurocentra in addition to the neural arch. The occipital. Fig. 142.—£ryops megacephalus, a rachitomous amphibian of the North Ameri- can Permian. Restoration based on mounted skeleton. condyle was triple or double and the interpterygoid vacuities wider than in the Embolomeri. The tabulars and dermo- supraoccipitals in this group form occipital flanges, not present in the Embolomeri. Watson (1919) recognizes the following fam- ilies: Eryopidae, Actinodontidae, Rhinesuchidae, Achelomidae, Dissorophidae, Trematopsidae, Zatrachydae, Archegosauridae, Trimerorachidae, Lydekkerinidae, Micropholidae, and Dwina- sauridae. The last mentioned, to judge from its hyoid was apparently a neotenous group. The best-known rachitomous amphibian is Eryops (Fig. 142). It attained the length of 4 or 5 feet and resembled an alligator but had a shorter tail. SUBORDER 3. Stereospondyll—The most advanced sub- order of labyrinthodonts lived during the Triassic period. They differed from the preceding in having the centrum formed almost entirely of the intercentrum, the pleurocentra being rudimentary or absent. Tendencies found in the Rachitomi are car
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