TORTOISES AND TURTLES. 



101 



as unlike bone as it can well be. This apparent dis- 

 crepancy can, however, be very readily explained. In 

 a living tortoise, as is 

 well shown in Fig. 27, 

 the outer surface of 

 the shell is completely 

 covered over with a 

 series of large shield- 

 like horny plates, of 

 which there is one 

 row down the middle 

 of the back, and a 

 lateral row on either 

 side of this middle one ; 

 externally to which we 

 have a series of rnar- 

 ginal shields, forming 

 the border of the 

 upper half of the shell. 

 Similar horny shields 

 also cover the lower 

 part of the shell, which 

 we shall notice shortly. 

 The solid bony shell 

 underlies these horny 

 shields; the relations 

 of the horny and bony 

 constituents of the protecting shell being shown 

 in Fig. 28, where the outer horny shields have 

 been stripped off, leaving distinct grooves on the 



