INTRODUCTION. 5 



reason is better adapted to the purposes of the physiolog-ist ; hence 

 I have chosen it for descrij)tion. 



The head is flat, as broad' as it is ' long'^ and triang-ular with an 

 obtuse snout in front. The upper surface of the head, i e. the 

 space between the eyes, is slightly concave, grooved, and narrower 

 than in E. temporaria. The tympanic membrane is circular, and 

 relatively to the eye is larger. The upper eyelids have several trans- 

 verse folds in their hinder part. The pujsil is oval, with the long 

 axis horizontal. The vomerine teeth are arrang-ed in two clusters, 

 which are relatively larger than in 11. temporaria and lie exactly 

 between the posterior nares, without however touching them. 

 The openings of the Eustachian tubes do not exceed in size the 

 posterior nares to so great an extent as they do in R. temporaria. 

 The male possesses a vocal sac on either side, which reaches the sur- 

 face beneath the tympanic membrane through a cleft placed behind 

 the angle of the mouth, and is, in well-developed specimens, about 

 the size of a cherry. The hind limbs are relatively longer. The 

 toes are long, and taper towards their tips : the webs between the 

 toes are cut out semicircularly, and that of the longest or fourth 

 toe is continued to the tip of the last phalanx. The supj)lemental 

 toe is an oval prominence of cartilaginous hardness. The skin 

 of the back has wart-like tubercles arranged long-itudinally in 

 raised lines; one of these lines runs on each side from the posterior 

 canthus as far as the thigh, and is very constant : in the male 

 a second line surrounds the posterior margin of the vocal sac ; 

 a corresponding line exists in the female. 



The skin of the belly is quite smooth, the colour i)resentiug- 

 many variations which appear to depend upon very diverse circum- 

 stances. It varies with changes in the physiological condition of the 

 animal. Von Wittich ^ has shown that a bright green specimen 

 changes to a dark leafy green colour on exclusion of light; also, that 

 dark specimens become almost a lemon-yellow colour on exposure to 

 bright sunlight; and he has pointed out that this brightening of 

 the skin is an active condition dependent upon contraction of the 

 stellate pigment-cells. It is therefore not surprising, as the same 

 inquirer observes, that one should sometimes find specimens of 

 R. esculenta in which the ground colour is almost a greenish yellow 

 (as in RoseFs figure, PI. XIII), whilst in others it can only be 

 distinguished from the dorsal black patches by a faint greenish 



* Vou Wittich, Miiller's Archiv, 1854, p. 41. 



