296 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. xxxvi. 



proaches the former more nearly in size, it resembles the latter in 

 the comj)arative lightness of its skeletal structure. 



The cervical vertebra* of 0. hrotcni, when compared with those of 

 C. dispar^ show a much narrower ventral keel and deeper lateral 

 depressions. The dorsal region, on account of the lack of material, 

 I am unable to contrast with the other species, excepting C. nanus., 

 the dorsals of which may be readily distinguished on account of their 

 small size and the thinness of the spinous processes, which are with- 

 out decided thickening of their superior terminations. 



The last, or sacro-dorsal, has a short but decided ventral keel which 

 at once distinguishes this element from the smoothly rounded haMnai 

 surface of this vertebra? in C. cJisfai\ or the faint keeled ones as found 

 in C. depressiis., G . nanus., and C . prestwichii. 



The sacral region of C. hroivni resembles O. depressus most nearly 

 in having seven vertebra* united by suture, of which five are con- 

 sidered true sacrals in the former species. The sacrum differs from 

 C. dispar in the increased number of sacral vertebra*, the absence, in 

 the anterior elements, of the peg-and-notch articulation, and its con- 

 tinuance into the caudal region, and by the more compressed and 

 keeled anterior centra. The absence of the peg-and-notch articula- 

 tion in the sacrals of 0. nanus and G. prestwichii at once separates 

 them from G. hroumi. 



Outside of the anterior caudals, as noted above, the other vertebrae 

 of the tail show no distinctive features. 



As known at the present time, the ilium is one of the most char- 

 acteristic bones of the entire skeleton of Gamptosauriis., and since 

 this element is present in all of the holot3'pes of the American species, 

 excepting G. amplus.^ it oifers a basis of comparison equal in impor- 

 tance to the differences displayed in the several species. The length 

 of the ilium (see fig. 30), which wants the extremity of the pre- 

 acetabular process, is quite equal to that of the average adult indi- 

 vidual of G. dispar (see figs. 1 and 3, Plate 14), although its greatest 

 dej)tli is considerably less. The preacetabular process when complete 

 was long, the superior border slightly convex, and the post-acetabular 

 portion long and especially narrow, the angular oblique border of 

 the supero-posterior end being short. Compared with the ilia of 

 the other species, the difference in the form of the hinder half, 

 coupled with its other proportions, would, apart from other evidence, 

 indicate the specific distinctness of its owner. In contour the ilium 

 is intermediate between G . medius on the one hand and C. depressus 

 on the other, as may be seen by comj^aring figures. Compared with 

 G. depressus.^ it shows a much wider preacetabular notch, a deeper 

 acetabulum, and a greater depth of the bone as a whole. 



The pathological condition of the right ilium of G. hroivni is of 

 interest in showing to what extent the shape of a bone may be modi- 



