VI.] 



THE COMMON FROG. 



63 



Moreover the bony plates of tortoises and turtles are 

 invested by large horny epidermal scales ("tortoise- 

 shell ") which scales do not agree in either size or 

 number with the bony plates on which they are 

 superimposed (fig. 26), 



Again, the middle series of bony plates of the back 

 are continuous with the subjacent joints of the back- 

 bones, and the lateral series of dorsal plates are con- 

 tinuous with the ribs beneath them (fig. 28 ) 



Fig. 29. — A Mud-tortoise {Trionyx), showing the dorsal plates. 



There are certain Chelonians, however — "mud- 

 tortoises " — (of the genus Trionyx), which have the 

 dorsal plates much less developed and not connected 



