FOSSIL BIRDS. 155 



fact, of every bird is composed in a very peculiar manner, and 

 resembles that of no other animal whatever. The birds are 

 the only class in which there is but a single bone to answer the 

 purposes of the tarsus and metatarsus. In the horses, the 

 metatarsus consists but of a single piece, whereas the tarsus 

 contains many. In the jerboa and alactaga there is also but a 

 single bone of the metatarsus, which supports the three prin- 

 cipal toes. But the bones of the tarsus are distinct. In the 

 tarsiers and galagos, the scaphoid and calcaneum are elongated, 

 so as to give as much length to the tarsus as in certain birds. 

 But the other bones of the tarsus and metatarsus do, neverthe- 

 less, equally exist. Frogs, toads, &c., also have the tarsus 

 considerably elongated. But it is always composed of two 

 long bones and several small ones. 



In the number of toes and of articulations on each toe in the 

 birds, characters are found no less striking than those furnished 

 by the tarsus. 



The birds are the only class in which the toes are all differ- 

 ent in the number of their articulations, under which never- 

 theless this number and the order of the toes are fixed. The 

 thumb, for instance, has two articulations, the first toe of the 

 internal side three, the middle toe four, and the external five. 



To this rule there is but one exception, and that is in the 

 case of birds which have no thumb ; but the other toes pre- 

 serve the usual number of articulations. 



This rule is never completely observed, excepting in this 

 class. 



The quadrupeds have two articulations on the thumb and 

 three on the other toes, be the number of the toes what it 

 may. The tridactyled sloths alone have but two, because the 

 first phalanges are cemented with the bones of the metatarsus. 

 Some few toes, indeed, that are concealed under the skin, want 

 the usual number. 



In the reptiles the number of articulations is less equal, but 

 scarcely ever do we find them exactly the same as in the birds. 



