FOSSIL BIRDS. 169 



The only quadrupeds in which any thing analogous may be 

 suspected, are those which, like the birds, rest and leap on 

 their hinder feet, with the body in an oblique position ; such 

 are the kangaroos and jerboas. 



We do find, in fact, in the various kangaroos a slight inden- 

 tion in the rear of the condyle to which the peroneum corre- 

 sponds, but only by a tubercle. The jerboas have not this 

 conformation. In the helamys of the Cape, indeed, there is a 

 particular osselet, which establishes a connexion between the 

 peroneum and the femur, but not in the same manner. 



There are moreover many traits which must prevent us from 

 ever confounding the femur of a bird with that of a kangaroo, 

 as well as with that of any other quadruped. Such, for ex- 

 ample, is the breadth of the great trochanter from front to 

 rear, &c. 



By the observation of these characters, two femora found in 

 the gypsum were clearly recognised to belong to birds. Their 

 cavity was filled with gypsous matter; they had not been 

 crushed by the weight of the strata deposited upon them, and 

 their forms were preserved perfectly entire. 



The tibia of birds is doubly characterised, by an upper head 

 corresponding to the form of the femur just described, and by 

 a lower one in the shape of a convex pulley, with a concave 

 neck, on which is articulated the bone of the tarsus. Many 

 such bones are found in the gypsum. The humerus of birds 

 is not less distinguished than the femur and tibia. 



The characters consist in the two extremities. In the upper 

 one the head is always oblong from right to left, to play like a 

 hinge in the articulation to which the shoulder-blade and 

 clavicle contribute. 



This part of the bone is singularly enlarged by two lateral 

 crests. The upper, or rather the external one, which is angu- 

 lar, and the edge of which is trenchant, and a little re- curved 

 in front, serves to give sufficient attachments to the great pec- 

 toral muscle, the powerful action of which constitutes the 



