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FOSSIL REPTILIA OP THE 



centre of motion, as in the pelvis of Grebes, Loons, Ostriches, and Emus. When the bird 

 slowly stalks, or hops, or climbs, or uses its legs chiefly in grasping and perching, the 

 pelvis is short and broad, especially behind ; its breadth may even exceed its length, as in 

 Cyclarius yuanensis." 1 



The antacetabular part of the ilium in Birds is usually the longest, but its outer 

 surface is not divided or interrupted by the super-acetabular plate and ridge peculiar to 

 Dinosaurs. To the degree in which the pelvis is produced behind the acetabulum (as in 

 woodcut, Eig. 14, b), such production helps to transmit the weight of the body upon the 

 legs in a relative position thereto more favorable to the support of such weight ; if the pubis 

 were directed forward instead of backward, it would detract from this relation of the pelvis 

 to bipedal progression. Nevertheless, the balance of the parts so carried in the Bird prepon- 

 derates forward ; the weight of the body with the head and fore-limbs is greatest in 

 advance of the acetabula. 



Among the modifications which are associated with the backwardly produced ilia, 

 ischia, and pubes, in relation to the terrestrial progression peculiar to Birds, may first be 

 noted the great extent of the axial trunk-bones welded into one mass where they are 

 grasped by the bones transferring such mass upon the heads of the femora. 



In no Birds are the sacral vertebrae so few as in Dinosauria ; and in those Birds which, 

 from their size and terrestrial habits, are cited to exemplify Dinosaurian affinities, and which 

 best lend themselves to test the question of the locomotion of the great extinct Reptiles, the 

 number of the sacral vertebrae is from 18 to 20. The several species oiDinornis had from 

 17 to 20 sacrals ; 12 is the average number in Natatores, 12 in Gralla: and Gattinacea, 

 11 in AUrices. The highest number of sacral vertebrae yet found in Dinosauria is 5 : 2 

 in Dicynodontia it is 6. The Sloths have 6 (Ai) or 8 (Unau) sacral vertebrae. The extinct 

 Megatherioids, from the great share taken by the massive hind limbs in supporting the 

 body while the fore limbs were engaged in disbranching trees, have a correspondingly closer 

 resemblance to Birds in the structure and proportions of their pelvis than any known extinct 

 Reptiles present. The Mylodon had not fewer than 11 anchylosed sacral vertebrae. 3 



In Birds, the trunk, properly so called, as distinguished from the neck, is singularly 

 short ; its production in advance of the pelvis is reduced to the utmost, consistently with 

 its visceral relations. 



The number of vertebrae between the neck and pelvis, i. e. of such as bear pairs of 

 moveable ribs, averages 8, and never exceeds 10 ; and of these anchylosis commonly 

 fetters the major part. 



Between such vertebrae and the skull the ' cervicals ' are as exceptional in excess, 



1 'Anatomy of Vertebrates,' vol. ii, p. 37. 



2 They may in an exceptional instance extend to 6, but demonstrative evidence of this excess has not 

 come to my knowledge. 



3 'Description of the Skeleton of an Extinct Gigantic Sloth,' &c, p. 64, pis. i, x, 4to, 1842. 



