354 



PROCEEDINGS OF THE NATIONAL MUSEUM. 



VOL. XXXV. 



Marsh " first attributed to Allosaurus fragilis was afterwards, without 

 expression of reasons therefor, referred 6 to Creosaurus atrox. Never- 

 theless, we can hardly suppose that Marsh ever regarded this vertebra 

 as a part of the type of A. fragilis; and, if we can depend on Marsh's 

 figure, said to be one-sixth the natural size, this vertebra is not the 

 one whose measurements are given by Marsh, inasmuch as its length 

 is 111) mm., instead of 105 mm. 



The type of Creosaurus atrox is a left ilium, which Marsh figured 

 more than once. c Marsh informs us in his earliest description that 

 there were some teeth which he referred to this species and which had 

 trihedral crowns and crenulated cutting edges; and he has figured 

 the right premaxillary with its five teeth. This premaxillary is prob- 

 ably a part of the individual that furnished the ilium. In the last- 

 mentioned citation Marsh stated that the anterior end of the ilium 

 might be imperfect. He gave, as the extreme length of the bone, 

 700 mm. It may be confidently assumed that the anterior border of 

 the bone had approximately the form seen in the ilium of Allosaurus d 

 and that the front of the pubic process has suffered some loss. Almost 

 certainly Marsh was mistaken when he suggested that a prepubic 

 bone had articulated with this process. The original length of the 

 ilium may be regarded as having been close to 720 mm. It is propose d 

 to compare some of its dimensions, taken from Marsh's figure, with 

 corresponding dimensions taken from the ilium, also 720 mm. long, of 

 the specimen of Allosaurus fragilis, now in the U. S. National 

 Museum, Cat. No. 4734, from which were made fig. 2, Plate X, and 

 fig. -J. Plate XI, of Marsh's Dinosaurs of North America. The second 

 column under each species presents the percentage which each 

 measurement is of the length of the ilium. 



Table <>f measurements mid percentages. 



Part measured. 



Greatest length of ilium 



Length of acetabulum 



Length from acetabulum to end of ilium 



Length from bottom of anterior notch to hinder end of 



ilium 



Width of bone where narrowest above acetabulum 



Height of upper border above acetabulum 



Allosaurus fragilis. 



Dimension. Pe r ^ nt " 

 age. 



Mm. 



720 

 ■±■12 

 836 



547 

 279 

 335 



100 

 31 

 47 



77 

 39 

 47 



< <reosaurus atrox. 



Dimension. 



Percent 



age. 



Mm. 



720 

 190 

 310 



530 

 300 

 320 



ioo 

 ■ic. t 



43 



73 

 42 

 44 



It will be observed that the differences in the measurements of the 

 two species do not reach 5 per cent of the length of the bone. The 



"Amer. Jour. Sci., XVII, 1ST!), pi. x, figs. 3, 4. 

 '' Idem, XXVII, 1884, p. 334, pi. xiv. figs. 3, 4. 



"Idem, XV, 1878, p. 243, figs. 1, 2; XVII, 187!), p. 90, pi. x, figs. 1, 2; XXVII, 

 1884, p. 335, pi. xiv, figs. 1, 2; Dinosaurs of N. Amer., 189<>, pi. xn. figs. 2, 3. 

 d Dinosaurs of North America, pi. x, fig. 2. 



