20 OLIVER P. HAY 



retained long after the disappearance of the fifth. In the lizards, 

 however, the fifth is often larger than the first, a condition depend- 

 ent perhaps on their habit of climbing about on rocks and trees. In 

 the mammals, on the other hand, it is the first digit that earliest 

 suffers reduction. 



An attempt has already been made on a previous page to account 

 for the origin of the bipedal habit in reptiles. Evidences are present, 

 it is believed, which show that bipedalism in the dinosaurs was not 

 due to specialization of the anterior limbs. If an examination be 

 made as to the relative lengths of the fore and the hinder limbs in the 

 carnivorous dinosaurs, it will be found that in Anchisaurus colurus 

 the fore limb is about three-fourths as long as the hinder; in Plateosau- 

 rus quenstedti about two- thirds; in Pachysaurus ajaoc, about one-half. 

 These are Triassic dinosaurs. In Ceratosaums, of the Upper Jurassic, 

 the fore limb is only about two-fifths as long as the hinder. In 

 Tyrannosaurus, of the Upper Cretaceous, the fore limb is diminutive, 

 in case the humerus found with the specimen really belonged to it. 12 

 As we have seen, the great pollex of the late Triassic forms had become 

 much reduced in the Upper Jurassic species. Therefore, in place of 

 specialization, the whole limb suffered degeneration. If now it be 

 asserted that bipedalism in the theropods was occasioned by speciali- 

 zation of the fore limb for other purposes than locomotion, we shall 

 have the case presented of an organ which, as soon as it was free to 

 continue its specialization, began to degenerate. Without doubt 

 however, the fore limb continued to be used for various purposes, just 

 as the ostrich continues to use its diminutive wings. 



Various opinions have been expressed regarding the origin of the 

 Sauropoda. Marsh 13 expressed the opinion that the group included 

 the most primitive forms of dinosaurs. Baur 14 held that the Sauro- 

 poda had no close relationships to the other reptiles usually classed 

 with them as dinosaurs. Osborn 15 believes that it is possible to 

 derive the sauropod type from a primitive quadrupedal theropod 

 type. In his work already so often quoted, Dr. v. Huene expresses 



12 Osborn. Bull. Amer. Mus. Nat. Hist., xxii, pi. xxxix. 



13 Dinosaurs N. A., p. 164. 



14 Amer. Naturalist, xxv, p. 450. 

 16 Nature, vol. 73, 1906, p. 284. 



