606 BRITISH FOSSIL REPTILES. 



excepted. The ilium in Pterodactyles yields the same ground as that of Dinosaurs for 

 predicating kinship with Birds. 



The true characteristic of the ilium in Binosauria is the distinction of the super- 

 acetabular (PI. 72, r) from the antacetabular (ib,. 62') parts of the bone, with the anterior 

 extension and subsidence, in some species, of the former upon the dorsal surface of the 

 latter. This complexity of the ilium is wanting in both birds and pterodactyles. 



As to the proportions of the ant- and post-acetabular extensions of the ilium, they 

 vaiy in \\iQys\\ JDinomuria : the post-acetabular production (PI. 72, 02") is shorter in 

 Omosaurus than in Scelidoscmrus, and is shorter in Scelidosaurus than in J(juanodon. 



From the importance assigned by Professor Huxley to iliac characters, in the con- 

 clusion he advocates, a non-anatomical reader might infer not only that no other Reptiles, 

 but that no other warm-blooded Vertebrates save Birds, had the ilium extended, as in 

 Dinosaurs, far in front of the acetabulum. 



And yet an impartial quest of the affinities of these huge terrestrial Reptilia would 

 impel the seeker, having such end solely in view, so to extend his comparisons. In 

 Mammals " the ilium is prolonged in front of the acetabulum," which, as in Reptiles, " is 

 either wholly closed by bone or presents a fontanelle." 



In the spiny Monotremes (woodcut, tig. 15, Ech'uhia) the ilium [g) extends far in front of 

 the acetabulum (A), and furnishes only an arched roof of that cavity, the inner wall of 

 which (0 remains membranous, as in the Bird. The pubis (a), after extending haemad 

 (forward or downward) to the pectineal process (c), bends there to be continued back- 

 ward, as in OrnithorhjncJius. As a rule all Mammals resemble Birds in a backward 

 extension of more or less of both pubis and ischium, from their iliac articulations. 



Thus the character asserted to be peculiar to Binosauria among Reptiles exists in 

 both the Pterosaurian and Crocodilian orders of that cold-blooded class ; and, amongst 

 warm-blooded Vertebrates, it is common to Mammals with Birds. 



In my ' Anatomy of Vertebrates ' I remarked, " the transference of the weight of a 

 horizontal trunk upon a single pair of legs necessitates an extensive grasp of the trunk- 

 segments. When the legs require to be pulled far and strongly back, as in diving and 

 cursorial motions, the origins of the requisite muscles are extended far behind the limb's 

 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 guanensisr* 



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

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

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

 woodcut. Fig. 14, 6), 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 



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



