190 REPTILES AND BATRACHIANS 



large, are protected by three lids, the upper, the lower, 

 and the nictitating membrane, which moves vertically, 

 and is of special use to the land forms when under water. 

 In the case of some of the tailed, and one or two of the 

 purely aquatic Tailless Batrachians, the eye is small and 

 devoid of lids, while in cave-dwelling Urodeles, Proteus, 

 Typhlomolge, and Typhlotriton, the eye is rudimentary or 

 quite invisible superficially. The pupil, which is more or 

 less contractile, is round or horizontal in the Urodeles, and 



© © © 



A 



® (I) ® ® 



B C D X 



Fig. 10. — Different forms of pupils. 



a. Bombinator pachi^pus. b. Alytes obstetricans. 



c, Pelobates fuscus, d. Rana arvalis. 



e. Bufo "vulgaris. 



(^After Boulenger. ) 



round, triangular, horizontal, or vertical in the Anures, 

 when contracted, such as depicted on fig. lo. The ear is 

 rudimentary in the Apoda, the Urodela, and in a few of 

 the Anura ; in the majority of the latter it is represented 

 externally by a circular membrane, the tympanum, or ear- 

 drum, which is situated behind the eye. The mouth is 

 usually large and cleft to beyond the eyes ; the upper or 

 lower jaw, or both, as the case may be, are armed with 

 numerous closely set, sharply pointed teeth, or teeth 

 are entirely absent ; the palate is always pierced by a 



