OSTEOLOGY OF THE ARMORED D1N0SAURIA. 



65 



Sternal bones. — Among the puzzling elements found associated with the skele- 

 tal remains of Stegosaurus in Quarry 13 were three bones (Nos. 7618, 7619, and 

 7620) that are here regarded as representing sternal elements. They are of special 

 interest, as this is the first time ossified sternal hones have been found associated 

 with Stegosaurian remains. 



These plates are small, triangular, asymmetrical bones. As I interpret these 

 elements (fig. 32), after carefully comparing them with the articulated sternals 

 of Tracliodon annectens (fig. 34), the broad truncate end is anterior, the pointed 

 end posterior. The transverse width of the anterior end is over one-half the 

 longitudinal length of the bone. This end is truncated with a comparatively 



Fig. 32.— Left sternal bone, Stegosaurus sp. Cat. no. 7620. U.S.N.M. ; nat. size. 1, Ventral view, a, Anterior 

 end; t, r, roughened surfaces for attachment of cartilaginous ribs. 2, Superior view, c, Point of cartilagi- 

 nous UNION WITH OPPOSITE PLATE. 



thin but roughly rugose border. On the inner side the anterior third presents a 

 thickened area winch looks upward and inward and doubtless represents the median 

 union of the plates of opposite sides. The ventral surface is slightly convex trans- 

 versely, and the dorsal or visceral surface is concave. Both surfaces are compara- 

 tively smooth with the exception of roughened areas (r and r, fig. 32) which probably 

 represent the points of attachment of the sternal ribs. The outer border is some- 

 what thickened and rounded, while the inner thins out to a sharp edge. Both 

 borders are slightly concave from end to end. 



Articulated, the broad truncated ends probably meet the anterior border of 

 the coracoids, and the elements of opposite sides join on the median line at the thick- 



