226 



REPTILIA 



CLASS III 



Fig. 32(3. 



Pelvis lit' Igiutnodon, l /20- ", Acetabulum; il, Ilium; is 

 Ischium ; o, Obturator process ; p, Pubis ; p' ', Postpubic process 



parallel with the almost equally slender but somewhat longer ischium. Although 

 this post-pubic process is suggestive at first sight of the pubis of birds, it has 



been demonstrated by the em- 

 bryological researches of Bunge 1 

 and Mehnert 2 that no homology 

 between these structures exists. 

 The avian pelvis is in its earliest 

 stages of development triradiate, 

 like that of the primitive Thero- 

 podous Dinosaurs, and the pubis 

 is directed forwards as in rep- 

 tiles. Later the pubis becomes 

 secondarily shifted backwards, 

 parallel with the ischium, and 

 the processus iliopedineus must be 

 regarded as a secondary struc- 

 ture peculiar to birds, with 

 which the pubis of Dinosaurs 

 has, of course, no homology. 

 The post-pubis of the Predentata 

 is accordingly to be interpreted as a special or adaptive modification, probably- 

 correlated with the function of the tail in balancing the body in locomotion, 

 or while resting on its hind quarters. 



The femur usually exceeds the humerus in length, and is placed vertical 

 to the body, with its head extending inward into the acetabulum, as in birds. 

 Dollo has shown that the femur of Iguanodon, with its large fourth trochanter 

 (a process which is distinct from the " third trochanter " of mammals), is con- 

 structed more upon the avian than reptilian type, and indicates a powerful 

 caudo-femoral musculature. Still other ornithic characters are exhibited by 

 the tibia, which possesses a large procnemial crest, and is often completely 

 embraced by the astragalus at its distal end. In the Stegosaurs these elements 

 are fused, but in most of the carnivorous Theropoda the astragalus bears an 

 ascending process, which is opposed to the anterior face of the lower end of 

 the tibia, exactly as in Pterosaurs and young Batite birds. The fibula is 

 complete, the proximal tarsals are two in number (astragalus and calcaneum), 

 and the distal two or three, tarsalia i-lli, being usually fused into a single 

 piece. The hind foot is either tridactyl or pentadactyl, but all five metatarsals 

 are commonly indicated, even when the number of functional digits is 

 reduced. 



Dinosaur remains were first discovered in the English Upper Jura and 

 Wealden during the early decades of the last century, and were described by 

 Buckland and Mantell in 1825. Owen, in 1841, erected for them the order 

 1'inosauria, and much additional light was shed on their structure and relations 

 by Huxley, Marsh, and Cope prior to 1875. Avian resemblances were first 

 pointed out by Gegenbaur in 1864, who observed that the tibio-tarsus of 

 Comjpsognathus is decidedly bird-like. In this form also was detected the only 



Bunge, A., Zur Entwicklungsgesclnchte des Beckengiiitels der Ampbibien, Eeptilien mid 

 el. Dorpat, 1880. 



; Mehnert, /■:., Qeber die Eutwicklung des os pelvis der Vogel (Morphol. Jalirb. vol. XIII. 

 p. 255<), 1888. 



