130 THE BRITISH AMPHIBIANS. 



when mature, but some retain their gills throughout life, and some 

 never breathe by gills although they are present in the embryo. 

 Their skull articulates with the backbone by two condyles, which in 

 a few cases are absent. There are no sternal ribs and no true 

 sternum. The lirnbs, when present, are four in number and have 

 distinct digits. When fins are present they are without fin -rays. 

 They are divided into two sub-classes, one of which, Stegocephali, 

 consists entirely of extinct species in which the head was protected 

 by bony shields, the other, Lissamphibia, comprises those that have 

 no such protection. Of these there are three orders, Ecaudata, 

 Caudata, and Apoda. Of the last, or limbless Amphibians, we have 

 no representative. 



The Ecaudata have no tail when adult. They have four limbs. 

 The body is short ; there are only eight, sometimes only seven, pre- 

 sacral vertebrae. The frontal bones coalesce with the parietals, the 

 radius coalesces with the ulna, the tibia coalesces with the fibula. 

 The hind legs have an additional segment owing to the elongation 

 of the astragalus and calcaneum in the metatarsus. There are two 

 sub-orders, Aglossa and Phaneroglossa, the Aglossa being distin- 

 guished by the absence of a tongue. The Phaneroglossa are divided 

 into two series, Firmisternia and Arcifera, in the first of which the 

 ventral halves of the shoulder girdle abut against each other and are 

 firmly held by cartilage, while in the second the coracoid and pre- 

 coracoid on each side diverge and are joined by curved cartilages 

 which overlap each other. Of the Firmisternia the only British 

 family is Ranidai (the frogs), the Arcifera being represented by 

 Bufonidae (toads). 



The Ranidae have teeth in the upper jaw. There are no ribs. 

 The vertebrae are concave in front, and the transverse processes 

 of the sacral vertebra are either cylindrical or very slightly dilated. 

 The precoracoids are parallel with the coracoids and are ossified 

 from the clavicles. The only British genus is Rana (Plate xxx., 

 82, 83). 



The Bufonidae have no teeth in either jaw. There are no ribs. 

 The vertebrae are proccelous, and the transverse processes of the 

 sacral vertebra are distinctly dilated. The precoracoids are in 

 contact with the coracoids at their outer ends and diverge towards 

 the middle, where they are joined to them by the arched epicoracoid 

 cartilage. 



The Caudata have a tail throughout life. They have generally 

 four, but in some cases only two limbs. The body is long and like 

 that of a lizard, or that of an eel. The frontal bones do not 

 coalesce with the parietals, nor do the palatines coalesce with the 

 maxillaries. The radius is distinct from the ulna, and the tibia from 

 the fibula. The vertebrae are concave behind sometimes, as in 

 the British genus, concave at both ends and carry short ribs. 

 There are four families. In one, Sirenidae, the jaws are toothless 

 and there are no hind legs. The others have teeth in both jaws 

 and all four legs. In one, Proteidae, there are no maxillaries ; both 

 the other two have these upper jawbones, but one, Ainphiumidse, 



