88 BULLETIN 89, UNITED STATES NATIONAL MUSEUM. 



Phalangeal formula of known dinosaurian hind feet: 



Scdidosaurus harrisonii Owen 



Camplmaurus dispar Marsh 



Tbescclosaurus negltttus Gilmore 



Brachyceratops montanensis Gilmore 

 Jguanodon bernissartensis Boulenger . 



Trachodon anneetens (Marsh) 



Diplodocus carnegii Hatcher 



Brontosaurus eicelsus Marsh 



Digits- 



ni 



IV 



Fig. 51. — Metatarsal U of eight hind foot of 

 Stegosaukus tjngulatus Marsh. ' ( nat. size. 

 a, Side view; 6, fp.ont view. After Miesh. 



The more recent discovery of an articulated limb (No. 4936) throws consider- 

 able doubt on the above interpretation, and I am now inclined to the opinion that the 

 ungual does not belong to the digit as shown in figure 52, III, but was shifted into its 

 present position from one of the other toes, as shown in plate 21, figure 1. The 

 evidence for this is, first, the great resemblance of the short metatarsal to metatarsal I 

 of Sauropodous dinosaurs, and, second, the finding of a disarticulated foot in the rock 



near and around the distal ends of a tibia and 

 fibula which were articulated with the femur. 

 If this association is correct, it shows the short 

 metatarsal to be on the inside of the foot, and 

 would therefore be metatarsal I as indicated 

 in figure 52. Upon other evidence Marsh has 

 given this bone a similar position in the hind 

 foot, as shown in his restoration (pi. 32, upper 

 figure). From the great resemblance of this 

 short metatarsal to the homologous element in 

 the hind foot of Brontosaurus and Diplodocus, 

 I regard it as representing metatarsal I, and the rudimentary metatarsal I would 

 consider as the remnant of Digit IV. 



The loss of the fifth digit and the reduction of the fourth to a vestigal meta- 

 tarsal shows that the reduction in the pes of Stegosaurus has taken place from the 

 outside of the foot as in the Sauropodous dinosaurs rather than from both outer 

 and inner sides, as in some of the Ornithopoda. 



It may be that there is a progressive increase in the number of phalanges in 

 Digits I to III so characteristic of nearly all dinosaurian reptiles, but I am inclined 

 to believe that when the complete formula is known it will be found that Digit III 

 lacks the ungual and perhaps one or more of the phalanges. 



Metatarsal I is an exceedingly short, stout element, the width about equaling 

 the length. The proximal end is wider than deep, thus reversing the measurements 

 of the median metatarsal. Viewed from the front the whole bone is beveled off 

 toward the outside. The chief characteristics of this element are shown in plate 21, 

 figure 1, I. 



Metatarsal II may at once be recognized by the transversely compressed proxi- 

 mal end and the greatly lengthened antero-posterior diameter of this end (fig. 51). 



