The tree-frogs, Hylidae, with which the arboreal Ranidae were formerly grouped, show in their anatomical structure a close resemblance to the toads, Bufonidae, and are therefore placed far away from the true frogs, however great the superficial resemblance between them.
The family Ranidae embraces a large number of genera, some of which are very remarkable.
The curious horned frog of the Solomon Islands, Ceratobatrachus guentheri, which can hardly be separated from the Ranidae, has teeth in the lower jaw in both sexes, whilst a few forms, such as Dendrobates and Cardioglossa, which on this account have been placed in a distinct family, have no teeth at all, as in toads.
The former type is exemplified by the toads and the lower Ecaudata, whilst the latter is characteristic of the true frogs (Ranidae), although when quite young these batrachians present a condition similar to that which persists throughout life in their lower relatives.
The first would be characterized by the Caudata, which are almost confined to it (although a few species penetrate into the Indian and neotropical regions), the Discoglossidae, mostly Europaeo-Asiatic, but one genus in California, and the numerous Pelobatidae; the second by the presence of Apoda, the prevalence of firmisternal Ecaudata and the absence of Hylidae; the third by the presence of Apoda, the prevalence of arciferous Ecaudata and the scarcity of Ranidae, the fourth by the prevalence of arciferous Ecaudata and the absence of Ranidae, as well as b y the absence of either Caudata or Apoda.