10 INTEODUCTION. 



nately opening and closing as the throat is drawn in 

 and swollen out during respiration. 



The eye is large, lateral (Fig. 1, a), or directed 

 upwards and outwards (Fig. 1, b), and protected by 

 three lids : the upper thick and coloured, the lower 

 rudimentary and immoveable, and the nictitating 

 membrane, transparent or partially pigmented, which 

 ascends over the eyeball to meet the upper lid. The 

 eye can be covered without the vision being much 

 impaired by elevation of the nictitating lid, as when 

 the frog is under water, or completely closed when 

 the eyeball is drawn in and the upper eyelid descends 

 to meet the lower. The iris is brilliantly coloured 

 with metallic pigment, golden, silvery, or bronzy, 

 with or without an admixture of red or black. The 

 pupil when fully distended is round and large, but 

 when contracted affects various shapes, which afford 

 important characters for the distinction of the genera. 

 Thus the contracted pupil of liana, Hyla, and Bufo 

 is horizontal, oval with an angle in the lower border 



Fig. 2. 



® ® ® 



A 



15 C D E 



Different forms of pupils. A. Bomhinatorpachypus. B. Alyies 

 obstetricans. c. Pelohatesfuscus. i>. Rana arvalis. E. Bufo 

 vulgaris. 



in Rana (Fig. 2, d), and Btofo viridis and calamita, with 

 an upper and lower angle in Hi/Ia and Bufo vulgaris 

 (Fig. 2, e). That of Dlscocjlossus and Bomhinator may 

 be described as roundish or subtriangular according 

 to specimens, the lower angle being always distinct, 

 but the upper border sometimes convex, when the 



