Perhaps the least unsatisfactory way of distinguishing between _R. agilis_, _R. graeca_, and _R. latastei_ (_R. iberica_ need not be confounded with them on account of its distribution) is the size of the tympanum, and its distance from the eye. The tympanum is smallest in _R. graeca_, its diameter being about half that of the eye and from ¾ to the whole of its width distant from the eye. In _R. latastei_ the tympanum is a little larger, and about ½ to ⅔ its own width distant from the eye. _R. agilis_ has the largest tympanum, measuring about ¾ of the diameter of the eye, and the distance between the two organs amounts to only ⅓ of the size of the tympanum. Brown land-frogs of the _R. temporaria_ group are found in most countries of nearly the whole Periarctic and Oriental regions, and by the time their races and varieties have been studied as minutely as those of Europe are now being scrutinised, the number of species will indeed be great. _R. silvatica_ is the chief representative in North America. It closely resembles _R. agilis_, but is smaller, only 2 inches in length, {260}and possesses a pair of internal vocal sacs. Its specific name refers to its predilection for forests of oak, among the dried leaves of which it conceals itself so successfully that it is discovered with difficulty. _R. japonica_ of Eastern Asia is almost indistinguishable from this American species and from the European _R. agilis_. _R. opisthodon_ of the Solomon Islands has the vomerine teeth in two oblique series entirely behind the level of the choanae. The general shape of this large frog is stout, the type specimen of the male measuring 78, that of the female 125 mm. = 5 inches. The upper surface of the female is covered with small, flat warts, that of the male is much smoother. The upper parts are dark brown, while the under surface is brownish white. The male has two internal vocal sacs.