LIFE AND KINSHIP OF DINOSAURS. 607 



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 verteljiaj is from 18 to 20. The several species olDinoniis had from 

 17 to 20 sacrals ; 12 is the average number in Natatores, 12 in Grallce and GaUinacea, 

 11 in Altrices. The highest number of sacral vertebrae yet found in Dinosauria is 5 :* 

 in Bicynodontia 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 Ilylodon had not fewer than 11 anchylosed sacral vertebrae. f 



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 l)ear pairs of 

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

 fetters the major part. 



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

 numerically ; and this concurs with the exceptional reduction of number in the ' dorsals ;* 

 both being in special physiological relation to bipedal support and progression. 



The numerous cervicals have peculiar joints, governing the sigmoid flexure and 

 oscillating sway of the long and slender neck ; whereby, in walking, both neck and head, 

 in Birds, may be brought more directly over the supporting columns of the hind limbs as 

 these change their position. These limbs, moreover, have their specialties in relation to 

 their peculiar work in the vertebrate series. 



The femur (Fig. 14 {Dinortns), f) is relatively short; the tibia {t) relatively long; the 

 fibula {fb), styliforra and short, takes no share in the ankle-joint, but co-operates with the 

 tibia in a special manner to extend and strengthen the articulation of the leg with the thigh. 

 The femoral condyles are concomitantly modified to effect the accessory femoro-fibular 



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

 come to my knowledse. 



t 'Description of the Skeleton of an Kxtiiict Gigantic Slotii,' &c., p. G4, pis. i, x, 4to, \t^\2. 



14 Ij 



