86 



BULLETIN 89, UNITED STATES NATIONAL MUSEUM. 



Hind foot. — The proximal row of the tarsus is composed of the astragalus and 

 calcaneum, the distal row apparently remaining unossified. In adult individuals 

 the astragalus usually coossifies with the tibia; calcaneum generally remaining as 

 a distinct element, although in two instances it was found ankylosed with the 

 fibula. 



A complete articulated hind foot of Stegosaurus is as yet unknown, although 

 associated foot bones enable me to determine the chief features of the structure of 

 the pes. 



In the hind foot there are three functional digits, with the rudiment of a 

 fourth. The latter, as first determined by O. C. Marsh, was thought to represent a 



remnant of digit one, but, as I shall attempt 

 to show, probably represents the fourth. 

 The fifth appears to be entirely wanting. 



In the first restoration of the hind foot 1 

 Marsh showed five digits, evidently drawn 

 from a fore foot, but this error was corrected 

 by him in a later restoration. 2 



Astragalus (a.). — The astragalus in Steg- 

 osaurus is reduced to a flattened, disk-like 

 bone, having a regular concave plantar sur- 

 face and an irregular convex proximal sur- 

 face. Viewed from above it is suboval in 

 outline, the longer diameter being transverse. 

 When articulated, the end of the tibia more 

 than covers it (see fig. 48). Externally it 

 meets the inner border of the calcaneum, with which it is often ankylosed. 



The median superior surface is covered with rounded protuberances (fig. 49), 

 which fit into corresponding depressions in the cupped surface of the distal end of 

 the tibia, thus rendering the two bones immovable upon one another. This joint, 

 as mentioned previously, is most favorable for their early coossification, a condition 

 observed in nearly all fully adult individuals. 



Although Marsh included this character in all of his definitions of the family 

 Stegosauridse, he was apparently well aware of its being an age characteristic, as 

 evidenced by the following: "In the Stegosauridae alone, among known dinosaurs, 

 is the astragalus coossified with the tibia. This, however, is not a character of 

 much importance." 3 



Measurements of astragali. 



Fig. 49.— Astragalus of Stegosaurus sp. Cat. no. 

 7387, TJ.S.N.M. 1 nat. size. Proxi ma l view. 





Stegosaurus, sp. 



No. 7378. 



No. 7387, 

 right. 



No. 6646, 

 right. 



No. 7380. 



Greatest diameter antero-posteriorlv 



mm. 

 104 

 110 



mm. 

 95 

 112 



mm. 

 122 

 115 



mm. 

 76 



87 





' Amer. Joum. Sci., vol. 21, 1881, p. 169, pi. 8, fig. 2. 



"- Idem., vol. 42, 1891, p. 180, pi. 9. 



3 Idem, vol. 23, 1S84, p. 336. 



