_Hyla._–The pupil is horizontal. The tympanum is distinct or hidden. The tongue is entire or slightly nicked in its hinder margin, which is more or less free behind. The fingers and toes are provided with typical adhesive discs. This is the largest genus of all Amphibia, containing about 150 species, and its distribution coincides with that of the whole family. Many of the species are very closely allied to each {190}other, differing only in small points, for instance in the extent of the webs to the fingers and toes, the configuration of the vomerine teeth, the size and appearance of the tympanic disc, and the relative length of the hind-limbs. In some of the West Indian, and in one Brazilian species, _H. nigromaculata_, the upper surface of the head is rough, owing to the cutis being involved in the cranial ossification. Bony or perhaps only calcareous deposits in other parts of the skin are rare, but are notably developed in _H. dasynotus_ of Brazil, in which they extend from the head to the sacrum, rendering the skin immovable. Many are capable of changing colour to a great extent, and it is a popular error to suppose that all tree-frogs are green, although this colour is perhaps the most common in the arboreal kinds. [Illustration: FIG. 37.–_Hyla arborea_, var. _meridionalis_. South European Tree-frog, × 1.] _H. arborea._–The tongue is rather round, slightly nicked behind, and can be protruded but little. The tympanum is distinct, but small. The upper parts are grass-green, quite smooth and shiny owing to the skin being covered with a film of moisture; the under parts are yellowish-white and granular, flesh-coloured or rosy on the thighs. Total length of large females 2 inches. This, the Tree-frog of Europe, has an enormous range, namely, from Morocco, France, and the south of Sweden, across the whole of Europe and Asia Minor to Japan and Southern China. {191}Several varieties have been described: the _typical_ or _European form_ is ornamented with a narrow black stripe, which, beginning at the nose, extends backwards along the side of the body to the groin, where it generally forms a hook turned upwards. This black colour forms the ventral boundary of the green, and is itself narrowly seamed with white on its upper border. In the south of France, the Iberian Peninsula, Morocco, and the Canary Islands the black lateral stripe is often absent; this is the var. _meridionalis_. In Spain and Portugal both forms are found in the same localities. In the Asiatic, chiefly in the eastern specimens, the lateral stripes tend to break up into irregular spots, vanishing altogether towards the groins; this var. _savignyi_ s. _japonica_ occurs also on most of the Mediterranean islands.