Sub-Fam. 2. Raninae.–The vertebrae are procoelous and devoid of ribs. The precoracoids are always present and ossified from the clavicles, and are parallel with the much stronger and ossified coracoids. The omosternum usually possesses a bony style, but in the Indian genera Nannobatrachus and Nannophrys and in Phyllodromus of Ecuador it remains cartilaginous, and in Colosthetus of Colombia it is absent. The metasternum also possesses a bony style, but it remains cartilaginous in the Indian genera Oxyglossus, Nannophrys, Nannobatrachus and Phyllodromus, in the last two genera rather reduced and slender, while in the Ecuadorian and Colombian genera Hylixalus, Prostherapis and Colosthetus, it is reduced to a membranous piece. In quite a number of genera the normal number of phalanges is increased by one owing to the intercalation of an extra phalanx between the terminal and the otherwise penultimate phalanx.[[104]] This is the case in all the species of Cassina, Hylambates, Rappia, Megalixalus, Rhacophorus, Chiromantis, Ixalus and Nyctixalus, but it is doubtful if all these genera are thereby more nearly related to each other than to the rest of the Raninae. The structure of the tips of the fingers and toes exhibits more variety. The terminal phalanges are mostly simple, with slight swellings at the ends, or they are Y- or T-shaped in conformity with more or less developed adhesive discs; in the African genus Hylambates only they are claw-shaped, as in the Hylidae.
Gampsosteonyx batesi, recently described by Boulenger from the Gaboon, shows a unique modification of the terminal phalanges of the second to the fifth toes. They are transformed into sharp and curved claws, like those of a cat, but instead of horny sheaths, it is the bone itself which is thus sharpened and perforates the skin, an anomaly reminding us of the ribs of Triton waltli. Total length of the type-specimens, about 3 inches.
Adhesive discs are common, and are best developed in Rhacophorus, Ixalus, Rappia, and Megalixalus. In the Neotropical genera, excepting Colosthetus, the discs are very peculiar, being provided on the upper side with leathery scales which are separated by a fissure. The fourth and fifth metatarsals either diverge and are connected by a distinct web, or they lie close together with only a groove between them, or lastly they appear externally united.
Fig. 46.–Map showing distribution of the Ranidae.
The tympanic disc is very variable, large, small or quite hidden. Vomerine teeth are present or absent. The pupil contracts into a horizontal slit except in some Palaeotropical genera. The tongue is universally free behind, mostly deeply notched, and can be well protruded; only in the Indian Oxyglossus and in the Neotropical genera, excepting Hylixalus, its posterior margin is entire.–There are terrestrial, arboreal, and aquatic members in this large sub-family. The geographical distribution of the Raninae, which comprise about twenty genera with at least some 270 species, is almost entirely Arctogaean. None, with the exception of three species in the Papuan subregion, occur in the Australian region; and only four genera, with one or two species each, inhabit the tropical Andesian district, the remainder of South America being without any Raninae. All the species of the whole Periarctic region belong to the genus Rana except in Eastern Asia, where the closely allied genus Rhacophorus occurs also. The entire sub-family of Raninae is, in its fulness and diversity of development, essentially Palaeotropical.
Many of the genera, even in the present more liberal sense as interpreted by Boulenger, are based upon unimportant characters, and in reality run into each other. This is for instance the case with Rana and Rhacophorus.
The following tabular arrangement is merely a key for determination and does not necessarily express relationships. The presence or absence of vomerine teeth is a character easily ascertained, but it separates closely allied genera, for instance, Rhacophorus from Ixalus and Micrixalus from Rana.
The genera with extra, interpolated phalanges are marked *.
Key for the Determination of the genera of Raninae.