170 THE WONDERS OF GEOLOGY. Lect. II. 



incident to a long chase ; but in the cat tribe, they are 

 curved and sharp, and these qualities are preserved by 

 a particular mechanism. The last bone which supports 

 the claw is placed laterally to the penultimate bone of the 

 phalanx, and is so joined to it, that an elastic ligament 

 draws it back, and raises the sharp extremity of the claw 

 upwards ; and the proximal, or nearest extremity of the 

 farthest bone, presses the ground in the ordinary running 

 of the animal, while the claw is retracted into a sheath : 

 but when the creature makes a spring and strikes, the 

 claws are uncased by the action of the flexors or bending 

 tendons. In the Bengal tiger, the claws are so sharp and 

 strong, and the arms so powerful, that they have been 

 known to fracture the skull of a man, by a single touch 

 from the animal in the act of leaping over him. A cat 

 affords a familiar illustration of this peculiarity of struc- 

 ture ; when pleased, its claws are retracted, and when angry 

 they are thrown out. 



In the claw of the Megalonyx there is no such lateral 

 provision for its retraction, and the point could not have 

 been raised vertically, as in the cat, so as to have permitted 

 it to touch the ground without injury. The articulating 

 surface is double, that is, there is a ridge or spine in the 

 middle, and it must, therefore, have moved like a hinge. 

 The bones of the arm present a corresponding configuration 

 with those of the sloth ; the humerus is perforated for the 

 passage of the blood vessels, and the radius is constructed 

 for rotation. 



The Megalonyx resembled the Megatherium in its gene- 

 ral character, configuration, and habits, but was one-third 

 smaller than that gigantic creature.* 



* Casts of all the bones and teeth of the Megalonyx that are in the 

 Museum of Philadelphia, were presented to me by Dr. S. G. Morton of 

 that city, and are now in the British Museum, in the same case with 

 the models of the bones of the Megatherium. 



