Chap. II. THE CIIELYDROIDiE. 345 



On the legs some of those tubercles are cnlargetl, and their scales form sharp 

 pniijccting ridges ; along the top of the tail there is a row of very strong and 

 large tubercles of this kind, and there are many other large ones about the tail 

 generally, forming on some parts of it a continuous covering. 



The animal lives mostly in the water, but makes considerable passages over- 

 land. It does not, like the Trionychidas, remain burrowed in the soft muddy 

 bottom, but rather lies in wait for prey under shelving banks, or among the I'eeds 

 and rushes. It moves over the bottom with long strides, touching it with the 

 feet, and also striking the water with the broad surface of the feet and of the 

 legs. Both in the water and on dry land, the limbs move in a much more 

 nearly perpendicular plane than in the TrionychidiB, and the body is raised high 

 from the ground ; on (hy land, a considerable part of the weight of the body 

 thus raised is borne by the long, strong tail, which reaches down to the ground. 

 When the animal is at rest, the elbow is brought up and back, and a little inward ; 

 the forearm is turned down, and tlie flat of the foot rests on the ground ; the 

 knee is carried forward but little upward, the leg below the knee is turned back 

 upon the femui", and the foot again turned forward, resting on the ground ; the 

 neck is withdrawn so as to carry the back part of the head under the carapace ; 

 the tail is bent to one side. See PI. 4 and 5. In this position, the head, the 

 limbs, and the tail are ready for action, the hind pairs of Hmbs well protected 

 l)y their position under the body, and all withdrawn nearly as far as they can be. 

 When danger approaches, the animal does not try to withdraw its head and limbs 

 further into the shield, but resorts to a more active defence. It faces the attack, 

 raises itself upon the legs and tail, highest behind, opens widely the mouth, and, 

 throwing out the head quickly as far as the long neck will allow, snaps the jaws 

 forcibly upon the assailant, at the s<ame time throwing the body forward so pow- 

 erfully as often to come down to tlie ground when it has missed its object. As 

 far as regards the wall of the animal, this is almost the exclusive mode of defence, 

 for it is slow to retreat, and cannot withdraw entirely into the shield. It catches 

 its prey in a similar way, by throwing the head forward. 



Man}- of the most important di.stinguishing characters of this family may clearh" 

 be traced to its peculiar habits. For example, the height and exposed condition 

 of the front end, the descent of the shield behind, the po.sition of the limbs and 

 consequent form and small size of the plastron, the breadth of the hind part of 

 the head, the strengtii of the neck and of the longissimus donsi, the consequent 

 flattening of the upper surface over the latter, and the size of the tail ; indeed, 

 nearly all the prominent characters given above are plainly connected with the 

 most marked pecvdiarity in the mode of life of the family, namely, the defence 

 by action with the jaw.s, instead of a quiet retreat into the shield. 



44 



