OSTEOLOGY OF THE AKMOEED DINOSAUEIA. 



55 



In S. ungulatus and S. armatus, No. 4936, as shown in figure 22, the ends of four 

 ribs unite to form this yoke, while in S. stenops only three participate in its forma- 

 tion. 



Marsh laid considerable stress on the manner of the articulation of the sacral ribs 

 with their respective centra, and even went so far as to establish * the species 8. 

 duplex largely on these differences. He says: 



In the sacrum of tliis species each vertebra supports its own transverse process or rib as in the Sauro- 

 poda, Willie m S. ungulalus the sacral ribs have sliifted somewhat forward, so that they touch, also, the 

 vertebra in front, thus showing an approach to some of the Omithopoda. 



Fig. 23.— Pelvic region of Steggsadeus stenops Mabsh. Cat. no. 4S34, U.S.N.M. Type About -^ n\t size 

 Ventral vieav snow as found in situ. A, Acetabulum; Co, fifth caudal; c.r, caudal eib; cs, caudo-saceal' 

 D, dorsal vertebra, or presacral no. 23; F. left femur; F', eight femue; 11, ilia; Is, left ischium- /y right 

 ischium; P, left pubis; F', eight pubis; r, Rins of left side; r', Ems or eight shje; st, sf, sS, sacral vertebrae 

 one, two, and three; si, dorso-sacral or presacral no. 27; X, last free rib articulated with prfsace\l 



NO. 20; y, BUTTKESS from the FffiST SACRAL WHICH ABUTS AGAINST II.a'M. THE BEAD OF THE RIGHT FEMUK \ND 

 portion OF THE RIGHT PUBIS ARE REMOVED TO BETTER SHOW THE SACRAL VERTEBRAE. 



After comparing Marsh's drawing of the sacrum of S. ungulatus (fig. 22) with the 

 sacra m the National Museum the actual dift'erences in this respect are so slight it 

 appears to be entirely one of degree which may readily be accounted for on the 

 ground of relative age. 



■ Amer. Joum. Sci., vol. 34, pt. 9, 1S87, p. 416. 



