OSTEOLOGY OF CARNIVOROUS DINOSAURS. 



141 



Fig. 76.— Ungual phalanx right 

 foot of oeninthomimus af- 

 flnis, new species. cotype, 

 NO. 6107, U.S.N.M. ANTERIOR 

 and lateral views, £ nat. size . 

 (After Lull.) 



These caudals were compared with Ornithomimid vertebrae in the American 

 Museum of Natural History, and so far as as comparison is possible, because of their 

 incomplete preservation, they showed a very close resemblance in all essential 

 features, especially in the prolongation of the anterior zygapophyses, the flattened 

 ventral aspect, and in the much enlarged proximal end as contrasted with the 

 smaller distal extremity. The centrum of No. 5701 has a 

 greatest length of 68.7 mm.; No. 6116, a length of 67.5 mm. 

 Judging from the size of some of the foot bones in- 

 dividuals of Omiihomimus affinis must have attained a size 

 slightly in excess of the skeleton of Ornithomimus altus 

 shown in plate 35, figure 2. 



A single centrum, No. 8454, U.S.N.M. (fig. 78), was 

 provisionally referred by Lull to Priconodon, but after 

 comparing it with the type of Ornithomimus sedens Marsh, 

 I now provisionally refer it to the genus Ornithomimus 

 and tentatively to 0. affinis. Lull's original description and 

 discussion of this bone is as follows: 



From the Arundel blue charcoal clay of Contee, Maryland, comes specimen No. 3101 [8454 U. S.N.M.] 

 of the Goucher College collection, the centrum of a dorsal vertebra, unlike anything else in the 

 entire mass of Arundel material. It represents a young animal, since the neural arch had not yet coos- 

 sified with the centrum. The centrum is opisthocoelous, the anterior articular face being plane; the 

 posterior a moderately deep concavity. The bone tapers decidedly, as the anterior face is less than 

 four-fifths of the transverse diameter of the posterior. The sides are decidedly concave and meet in a 

 slight keel-like angle inferiorly. The neural canal is extremely narrow, especially at a point just in 



front of the mid-length of the centrum. There are several tiny irreg- 

 ularly placed foramina, which enter the centrum from the bottom of 

 the canal, the two anterior ones being separated by a slight ridge. 

 The irregular ridges on the articular face for the pedicels of the neural 

 arch are approximately parallel, only radiating at the anterior end of 

 the centrum. The bone appears to be solid, of fine cancellous char- 

 acter, with no trace of lateral depressions, such as one finds in Pleu- 

 rocoelus. This vertebra, which evidently came from the mid-dorsal 

 region, compares very closely with a vertebra of Stegosaurus ungula- 

 tus (cotype No. 1858, Yale Museum), except for size. The two ver- 

 tebrae differ in the much less relative depth of the posterior con- 

 cavity in Stegosaurus, and in the fact that the anterior and pos- 

 terior concavity in Stegosaurus, and in the fact that the anterior 

 Fig. 77.— Distal caudal vertebra of " . ' » ' . 



Ornithomimus affinis new species. ano - posterior faces are the same diameter m Stegosaurus; but there 

 No. 5701, U.S.N.M.(A) superior view is a similar, though not quite so marked, tapering of the pedicel 

 slightly restored; (B), lateral facets. The groove of the neural canal is wider in Stegosaurus, and 



seems to lack the constriction ; but it is quite probable that the neural 

 canal widened perceptibly vertically before narrowing again in the 

 neural arch of Priconodon. There is no trace of a keel-like ridge on 

 the lower side of the centrum of Stegosaurus, though the curve of the section is sharper here than on 

 the sides. In neither case is there a trace of the capitular rib facet on the centrum, which in Stegosau- 

 rus is high on the greatly elevated neural arch. 



In Plate 17 of the forthcoming monograph of the Stegosauria this bone is figured, together with a lon- 

 gitudinal section, showing its extremely dense cancellous tissue comparable to that of the centrum under- 

 discussion. From this comparison it seems reasonably sure that we have here the centrum of a stegosau- 

 rian dinosaur which will be provisionally referred to Priconodon. Whether it may be referred to P. crassus 

 or not is an open question, for the vertebra is much smaller than that of the adult Stegosaurus ungulatus, 



view. Both figures \ nat. size. 

 The restoration of the zygapoph- 

 yses TOO SHORT. 



