TYPICAL FROGS. 265 



leg. Moreover, the hind-limb obtains a kind of additional segment, owing to the 

 elongation of the calcaneum and astragalus in the ankle-joint, which form a pair 

 of long bones lying parallel to one another. As a rule, frogs and toads undergo a 

 lengthened larval period ; the " tadpoles," as shown in the figure on p. 262, having 

 a globular head and body, a fish-like tail, external or internal gills, and no limbs in 

 the first stages of their existence. The hind-limbs are the first to appear, and 

 after the front pair are developed the tail is gradually absorbed, upon which the 

 young for the first time leave the water. Represented by about a thousand species, 

 frogs and toads have a worldwide distribution, although more abundant in tropical 

 and subtropical than in temperate regions, and being especially numerous in India 

 and South America ; and it is not a little remarkable that some of the largest 

 forms are inhabitants of islands. From the nocturnal habits of the adults it is 

 frequently difficult to find out whether in any locality these reptiles are abundant 

 or the reverse ; but in the spring this may generally be ascertained by observing 

 the tadpoles in the rivers and points, since all of these show specific differences, to 

 the full as well marked as those in the adult. 



The Typical Frogs. 



Family Ranid^:. 



The typical frogs, together with four other families, constitute a suborder 



(Firmisternia), characterised by the presence of a tongue, and by the firm union 



of the two metacoracoid bones of the chest by means of a single cartilage uniting 



their free edges. From the other members of the group, the typical frogs are 



distinguished as a family by the presence of teeth in the upper jaw, and by the 



transverse processes of the sacral vertebra being either cylindrical, or but very 



slightly dilated at their extremities. These characters are sufficient to distinguish 



the typical frogs from the other families of the suborder ; but it may he added that 



the vertebrae are cupped in front and hollowed behind ; while there are no ribs ; 



and the terminal style of the backbone is articulated to the sacrum by two 



condyles. The terminal joints of the toes may be either simple or pointed, 



T-shaped, Y-shaped, or even claw-like ; the species in which these joints are thus. 



expanded having the soft parts similarly expanded and flattened. For a long time 



it was considered that the shape of the tips of the toes was connected with the 



mode of life of their owners ; and although this is so to a great extent, it is now 



ascertained that several of the species in which the toes are somewhat expanded 



are as aquatic as those in which they are pointed, and species presenting both 



modifications are included within one and the same genus. The typical frogs are 



divided into twenty genera, only two of which are noticed in this work. 



Under the general title of water-frogs may be conveniently 

 Water-Frogs. . ° . 



included all the members (some hundred and forty in number), of the 



genus Rana, to which belongs the common English frog. The distinctive characters 



of these frogs are to be found in the horizontal pupil of the eye; the more or 



less deeply notched and free tongue ; the presence of teeth on the vomerine bones 



of the palate ; the absence of webs in the toes of the fore-feet, and their presence 



