108 BULLETIN 89, UNITED STATES NATIONAL MUSEUM. 



and base of the tail, which were doubtless most strongly attached, retained their 

 natural positions. As decomposition progressed the other bones of the skeleton 

 either settled into the sand, as is true of those of the lower or left side where the 

 ribs of the body are found spaced much as in life, while those of the right side have 

 been carried into a pile above the backbone and against the upstanding hip bones. 

 That these elements were piled up by the action of the current appears to be indi- 

 cated by the fact that the longer axes of the bones of this side all he in the same 

 general direction. In other words, the direction of the force of the current was 

 diagonally across the longer dimensions of the skeleton, as indicated by an arrow 

 on the diagram (pi. 3). Thus were the bones laid down, and the final part of 

 the story necessitates a rapid covering of the skeleton by sand in order to make the 

 condition right for the petrifaction and preservation of the specimen. This sand 

 in the many years following was covered by other sediments until many thousands 

 of feet had accumulated. The great pressure brought about by the accumulated 

 weight of the overlying strata consolidated the loose sand into the hard sandstone 

 that until recently completely enveloped the fossil. 



Discussion. — Since this specimen is the basis for that part of the present paper 

 devoted to the osteology of Stegosaurus, where the skeletal parts have been de- 

 scribed in detail, it is only necessary to discuss here those characteristics wherein 

 it differs from the other known species. On account of the inadequate descriptions 

 of the type-specimens upon which the other American species are based and which 

 are preserved elsewhere, I am unable to contrast these species to the best advantage, 

 but such differences as have been found appear to indicate that other and more 

 important specific characters will be disclosed when those types are more fully 

 known and available for comparison. 



Typically the skeleton of Stegosaurus stenops represents an adult animal of 

 medium proportions, being less robust than the specimens upon which S. ungu- 

 latus and S. sulcatus are based. From S. ungulatus this species may at present be 

 distinguished chiefly by characters found in the sacrum and femur. 



In S. stenops the sacrals have a decided ventral keel, as compared with the 

 broad, rounded, keel- less centra of S. ungulatus; the nonparticipation of the caudo- 

 sacral rib in the formation of the sacracostal yoke and the much shorter spinous 

 processes of the sacrals are other distinguishing characters. 



The presence of a rudimentary finger-like lesser trochanter (a, fig. 45) on the outer 

 anterior face of the proximal end of the femur of S. stenops, and its relatively shorter 

 length distinguish it at once from the slender, elongate femur of S. ungulatus where 

 the lesser trochanter has entirely disappeared. (Compare 1 and 3, fig. 45.) 



From jS. sulcatus this species may be distinguished bj r its smaller size and 

 differences displayed in the dermal tail spines, as shown in figure 60. 



The genus Diracodon ' was founded by Marsh upon two imperfect maxillary 

 bones (No. 1S85, Yale University Museum). Later, from the study of additional 

 material, other characters were assigned this genus and a figure of the distal end 

 of the tail with spines in situ (No. 4288, United States National Museum) was pub- 

 lished (pi. 15). 



1 Amer. Journ. Sci., vol. 21, 1881, p. 421. 



