tiOC) THE MKCHANICAL FUNCTIONS. 



or cavity for the reception of the thigh bone, at the 

 Iiinder extremity of the slender ])ones above men- 

 tioned. This is the simplest possible form to which 

 the pelvis can be reduced, while it preserves its 

 attachments to the spine. It presents in this re- 

 spect a more advanced stage of developement than 

 that of fishes. 



The connexion of the bones of the anterior ex- 

 tremities with the spine is analogous to that which 

 takes place in Rays and Sharks : there being an 

 osseous belt formed by the scapula, clavicle, and 

 coracoid bone, with the latter of which the humerus 

 is connected. The sternum is large, and consider- 

 ably developed ; making some slight approach to 

 the expansion it receives in the Chelonia. The 

 radius and ulna are united into one bone ; the 

 bones of the arm and leg in general resemble in 

 their figure and connexions those of the higher 

 orders of Mammalia, to the type of which this 

 Older of reptiles evidently approximates. There 

 are five toes in the foot, with sometimes the rudi- 

 ment of a sixth : the anterior extremity has only 

 four toes, which are without claws. 



The necessity of employing the same instruments 

 for progression in the water and on land, is pro- 

 bably the cause which prevents their having the 

 form best adapted for either function. The hind 

 feet of the frog, being well constructed for striking 

 the water backwards in swimming, are, in conse- 

 quence, less capable of exerting a force sufficient 

 to raise and support the weight of the body in 

 walking ; and hence this animal is exceedingly 

 awkward in its attempt to walk. On a short level 



