EXTERNAL CHARACTERS. 17 



Fig. 7. 



c 



Limbs of Hyla arborea, showing variations in the proportion 

 of the tibia. The dotted line indicates the middle line of 

 the body. 



The great development of the hind limbs enables 

 these Batrachians to proceed by leaps ; there is, how- 

 ever, one exception among the European forms, viz. 

 Biifo calamita, in which the limbs are so short as to 

 render hopping impossible, and this toad progresses at 

 a running pace when pursued. The other extreme is 

 furnished by Bana agiiis, which owing to its extremely 

 lone; hind limbs is able to cover a distance of six feet 

 at one leap. The aquatic habits of a Batrachian can, 

 as a rule, be gauged by the extent of the web between 

 the toes : thus Bvfo calamita and Ah/tes, which have 

 only rudimentary webs, are very bad swimmers, and do 

 not venture far from the shore ; whilst Bana esculenta 

 and Bombinator, with their broadly- webbed toes, spend 

 nearly the whole of the active period in the water. 

 But there are exceptions : B debates, the toes of 

 which are completely webbed, sojourns but a very 

 short time in the water ; it is true that in this case 

 the web is probably of use for pushing aside the soil 

 in the process of digging. 



The following synopsis of the species is based 

 entirely on external characters, and applies to both 

 sexes. 



I. Tongue circular, entire, adherent, or slightly 

 free behind ; vomerine teeth behind the level of 

 the choanal ; first finger shorter than second 

 (Discoglossid^e). 



