111.] THE COMMON FROG. 27 



of the gill-opening. Two others were dorsal spines 

 placed one behind the other on the mid-line of the 

 back. These poison-organs are probably only used 

 for defence. They are formed, however, on the ver}^ 

 same type as are the poison fangs of vipers. Unlike 

 the latter, however, they are not modified teetli, nor 



Fig. 8. Fig. 9 



Fig. 8. — Vertical, Longitudinal Sectif^n. of the Poison-fang of a Serpent (after 

 Owen). ^, deep groove ; o, its ]o*er termination, which affords exit to the 

 poison;/, pulp-cavitv. Fig. 9 — Magnified Transverse Section of a Serpent's 

 Poison-fang (a ter Owen), g, groove round which the substance of the tooth 

 (containing /, the pulp-cavity) is bent : j, the point where the sides of the 

 tooth meet and convert the " groove" into what is practically a cmtral cavity. 



are they situated within the mouth as they always 

 are in poisonous serpents. • 



A Frog {Pelcbates fiisacs) which is common in 

 France (and which is interesting on account of the 

 form of its skull hereafter to be pointed out), though 

 really harmless enough, has a singular power of 

 making itself offensive. 



Both males and females of this species utter a kind 

 of croak, and both, if the thigh is pinched, produce 

 a sound like the mewing of a cat. At the same time 

 they emit a strong odour, which is like that of garlic, 

 and becomes stronger as the animals are more dis- 

 turbed. This emission not only affects the sense of 



