In the argument thus far we have considered terrestrial, adult amphibians, since it is only in these that either the normal middle ear and tympanum, or the opercular apparatus, is present. But among the urodeles several neotenic types occur (this term applies also to the perennibranchs). For most of these there is nothing about the otic region that would be inconsistent with derivation, by neoteny, from known families in which adults are terrestrial; for example, _Cryptobranchus_ could have had a Hynobiid-like ancestor. But this, as mentioned above, does not hold for the Proteidae, which possess an opisthotic of relatively large size, distinctly separate from the exoccipital and prootic. Either this bone is a neomorph, which seems improbable, or there has not been in the ancestry of this particular family an episode of reduction comparable to that seen in the terrestrial families, where there is an operculum instead of a normal opisthotic. Therefore the Proteidae probably are not derived from the general stem of other salamanders, but diverged sufficiently long ago that the bones of the otic region were reduced on a different pattern. They need not be removed from the order, but, in this respect, recognized as more primitive than any other existing Urodela or Anura. A recent paper by Hecht (1957) discusses many features of _Necturus_ and _Proteus_, and shows that they are remote from each other; his evidence does not seem to prove, however, that they were of independent origin or that they need be placed in separate families. VERTEBRAE AND RIBS Development of the vertebrae and ribs of Recent Amphibia has been studied by Gamble (1922), Naef (1929), Mookerjee (1930 a, b), Gray (1930) and Emelianov (1936), among others. MacBride (1932) and Remane (1938) provide good summaries. In this section reference will be made to the embryonic vertebral cartilages by the names used for them in these studies, although the concept of "arcualia" is currently considered of little value in comparative anatomy. [Illustration: Fig. 5. Development of Anuran vertebrae. Upper left, late tadpole of _Xenopus laevis_; lower left, same just after metamorphosis; upper right, diagram of general components of primitive Anuran vertebra. (After MacBride, 1932, Figs. 35, 38, 47D, respectively.) Lower right, section through anterior portion of urostyle, immediately posterior to sacral vertebra, in transforming _Ascaphus truei_ (original, from specimen collected on Olympic Peninsula, Washington). All × 20 approx. For explanation of abbreviations see Fig. 3.]