78 CLASS REPTILIA. 



dry places, and not well formed for swimming, are fond of 

 marshes. Mr. Bell, as we have already observed, before his 

 discovery of the two genera, Kinixys and Pyxis, imagined 

 that the natural transition from emys to testudo, was to be 

 found here by means of his Terrapene Europcea. But the 

 true connecting link between the two families he has since, 

 as we have seen, much more satisfactorily established. 



The important character of the group of which we now 

 speak, is the mobile structure of the sternum. This sternum, 

 in all of them, may be considered as consisting of three por- 

 tions : the posterior is that part which is covered by the two 

 posterior pairs of plates — the middle one, by the next pair — 

 and the anterior by the remaining anterior plates, which dif- 

 fer in number according as the foremost pair are either united 

 into one single plate, or have a small supernumerary one 

 interposed between them. 



There are three different modifications which take place in 

 this part. " In the first (the genus Kinosternon, Spix), the 

 middle portion or lobe is quite fixed to the sides, the anterior 

 and posterior lobes moving upon it. In the second form (the 

 genus SternothcBvus, Bell), the middle portion is fixed, as in 

 the other, and the posterior one also connected with it by 

 continuous bony union, the anterior lobe only being moveable. 

 In the third (constituting the genus Terrapene, Merrem), 

 the middle and posterior lobes are also immoveably con- 

 nected together, but forming a single moveable valve, with- 

 out any bony union with the upper shell, the anterior lobe 

 being also moveable on the same axis. The only connexion 

 between these two valves and the upper shell, is by means of 

 a strong ligament, becoming cartilaginous at the axis. 



" The hinge or connection between the valves is formed 

 by a sort of articular cartilage, allowing, by its elasticity, of 

 sufficient motion to enable the animal to open the shell, so 



