Among typical frogs (Fig. 4) the base, or otostapes, is present and bony, the extrastapedial process (extracolumella, or hyostapes) is usually cartilaginous, the dorsal process (processus paroticus) is of cartilage or ligament, but the other two attachments are absent in the adult. The exoccipital extends laterally, occupying the posterior face of the otic capsule. Between it and the otostapes is a small disc, usually ossified, the operculum, which normally fits loosely in a portion of the fenestral membrane, and is developed from the otic capsule. The opercularis muscle extends from this disc to the suprascapula, in many but by no means all families of Anura. [Illustration: Fig. 4. Diagram of middle ear structures in _Rana_ (upper figure, after Stadtmüller, 1936, and lower left after DeBeer, 1937), and _Ambystoma_ (lower right, after DeBeer, 1937); all × 4. For explanation of abbreviations see Fig. 3.] Among Urodela (Fig. 4) the middle ear cavity and tympanum are lacking, and the stapes (columella) consists of no more than its footplate and the stylus, which is attached to the border of the squamosal, thus corresponding to the "internal" process. In families in which individuals metamorphose and become terrestrial (Hynobiidae, Ambystomidae, Salamandridae, Plethodontidae), an operculum and opercularis muscle appear in the adult, just as in frogs, except that in Plethodontidae, the most progressive family, the operculum fuses with the footplate of the stapes. Among neotenous or perennibranchiate urodeles there is no separate operculum or opercularis. The evidence given by Reed (1915) for fusion of the operculum with the columella in _Necturus_ appears inconclusive, in spite of the great care with which his observations were made. On the other hand, _Necturus_ and _Proteus_ alone among living salamanders have a distinct opisthotic on the posterior wall of the otic capsule (Fig. 3), as do the Cretaceous _Hylaeobatrachus_ and the Eocene _Palaeoproteus_. Probably these Proteidae should be regarded as primitive in this respect, although many other features may be attributed to neoteny. There is a contrast between Anura and most Urodela in the relative positions of the stapes and facial nerve, as shown in DeBeer's (1937) diagrams. In the latter (_Ambystoma_) the nerve is beneath, and in the former (_Rana_) above, the stapes. Judging by figures of _Neoceratodus_, _Hypogeophis_, and several types of reptiles and mammals, the Urodela are exceptional. _Necturus_, however, has the nerve passing above its stapes, and this may be primitive in the same sense as the persistent opisthotic. There can be, of course, no question of the nerve having worked its way through or over the obstructing stapes in order to come below it in salamanders; rather, the peripheral growth of neuron fibers in the embryo must simply pursue a slightly different course among the partially differentiated mesenchyme in the two contrasting patterns.