io 4 ANIMAL LIFE PAST AND PRESENT. 



the southern hemisphere, there is an additional un- 

 paired shield (Fig. 30, i, gu) dividing the first or gular 

 (gu) shields of the plastron. This additional, or inter- 

 gular, shield, as it is termed, is very frequently of a 

 battledore shape, as in Fig. 30. 



In regard to the bones composing the upper shell, 

 or carapace, it will be observed, from Fig. 28, that 

 although they by no means correspond in contour 

 with the overlying horny shields, yet that a decided 

 general similarity of arrangement obtains between the 

 two series. Thus there is a single middle row of bones 

 corresponding to the middle row of horny shields, 

 although the bones are smaller and more numerous 

 than the shields. Similarly there is a series of lateral 

 bones on either side of the middle row ; while in like 

 manner the border of the carapace is formed of a series 

 of marginal bones corresponding very closely with the 

 marginal horny shields. The bones of the middle 

 series of the carapace are severally attached to the 

 summits of the spines of the back-bone, or vertebrae ; 

 while the lateral bones are nothing more than large 

 expanded plates lying on the ribs, to which they are 

 so completely welded that they are generally regarded 

 as part of the same bone. 



From this brief glance at the structure of the 

 Chelonian shell, we are enabled to understand one of 

 the most peculiar features of the organisation of these 

 remarkable reptiles. This peculiarity is, that whereas 

 the shoulder-blade in all other animals lies entirely on 

 the outer side of the ribs, in the tortoises and turtles 



