Mr. Bell on the Box Tortoises. 5&i 



have, interposed between them, a small supernumerary one, as in 

 S\ Leachianiis, 



From these observations it will be easy to understand the three 

 different modifications which take place in this part. In the 

 first, the genus Kmustertioii^ the middle lobe is quite fixed, to the 

 sides, the anteriour and posteriour lobes moving upon it, by means 

 of the structure about to be described. In the second form, the 

 genus Sternothcerus^ the niiddle portion is fixed as in the other, 

 and the posteriour one also connected with it by continuous bony 

 union ; the anteriour lobe only being moveable. In the third, 

 constituting the genus Terrupene^ the middle and posteriour lobes 

 are also immoveably connected together, but forming a single 

 moveable valve, without any bony union with the upper shell, — 

 the anteriour lobe being also moveable on the same axis. The 

 only connection between these two valves and the upper shell, is 

 by means of a strong ligament, becoming cartilaginous at the axis. 



The hinge, or counecliori 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 as to move its 

 limbs without inconvenience, or, on the other hand, to bring it 

 into close contact with the upper shell, and thus to enclose iti^elf, 

 particularly in tJie genus Terrapene^ within a complete box. At 

 the angles of these valves are small processes of bone, or at least 

 distinct muscular impressions, to which the adductor muscles are 

 fixed ; and these, in the anteriour valve of Siernotheerus Leachir 

 anus form long spinous prgcesses. It is obvious that in the genus 

 Siernotheerus y the hinder part of the shell cannot be closed, as tha]t 

 part of the sternum is immoveable. . ; - 



Upon the whole then, notwithstanding the affinities by which 

 these animals are connected with the Emydes of Merrem, are such 

 as to forbid me to consider them as a distinct family, yet the 

 structure which I have been describing is so striking, and appears 

 to me of so much consequence, especially as requiring a consider- 

 able addition to, or modification of, the muscular system, that I 

 could not look upon it as forming a less important group than a 

 subfamily, particularly as it includes several subordinate divisions, 

 with distinct generic characters. 



