REPTILES. 397 



Frogs and toads offer one interesting sexual difference, 

 namely, in the musical powers possessed by the males; but 

 to speak of music, when applied to the discordant and 

 overwhelming sounds emitted by male bull-frogs and some 

 other species seems, according to our taste, a singularly 

 inappropriate expression. Nevertheless, certain frogs sing 

 in a decidedly pleasing manner. Near Rio Janeiro I used 

 often to sit in the evening to listen to a number of little 

 Hylae perched on blades of grass close to the water, which 

 sent forth sweet chirping notes in harmony. The various 

 sounds are emitted chiefly by the males during the breed- 

 ing-season, as in the case of the croaking of our common 

 frog.* In accordance, with this fact the vocal organs of the 

 males are more highly developed than those cf the females. 

 In some genera the males alone are provided with sacs 

 which open into the larynx, f For instance, in the edible 

 frog (Rana esculenta) "the sacs are peculiar to the males, 

 and become, when filled with air in the act of croaking, 

 large globular bladders, standing out one on each side of 

 the head near the corners of the mouth." The croak of 

 the male is thus rendered exceedingly powerful; while that 

 of the female is only a slight groaning noise. J In the sev- 

 eral genera of the family the vocal organs differ considerably 

 in structure, and their development in all cases may be 

 attributed to sexual selection. 



REPTILES. 



Clielonia. Tortoises and turtles do not offer well-marked 

 sexual differences. In some species the tail of the male is 

 longer than that of the female. In some the plastron or 

 lower surface of the shell of the male is slightly concave in 

 relation to the back of the female. The male of the mud- 

 turtle of the United States (Chrysemys picta) has claws on 

 its front feet twice as long as those of the female; and 

 these are used when the sexes unite. With the huge 

 tortoise of the Galapagos Islands ( Testudo nigra) the males 



*Bell, "History of British Reptiles," 1849, p. 93. 

 f J. Bishop, in " Todd's Cyclop, of Anat. and Phys.," voL iv, p. 

 1503. 



555 



{Bell, ibid, pp. 113-114. 



Mr. C. J. Maynard, "The American Naturalist," Dec. 1869, p. 



