OSTEOLOGY OF THE ARMORED DINOSAURIA. 



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 1 the species S. 

 duplex largely on these differences. He says: 



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

 poda, while in S. ungulatus the sacral ribs have shifted somewhat forward, so that they touch, also, the 

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



Fig. 23.— Pelvic region op Stegosaurus stenops Marsh. Cat. no. 4934, U.S.N.M. Type. About -fa nat. size. 

 Ventral view shown as found in situ. A, Acetabulum; C5, fifth caudal; c.r, caudal rib; cs, caudo-sacral; 



D, DORSAL VERTEBRA, OR PRESACRAL NO. 23; F, LEFT FEMUR; F' , RIGHT FEMUR; J?, ILIA; Is, LEFT ISCHIUM; iV, RIGHT 



ischium; P, left pubis; P' , right pubis; r, ribs of left side; r', ribs of right side; si, s;>, s3, sacral vertebrae 



ONE, TWO, AND THREE; si, DORSO-SACRAL OR PRESACRAL NO. 27; X, LAST FREE RIB ARTICULATED WITH PRESACRAL 

 NO. 26; y, BUTTRESS FROM THE FIRST SACRAL WHICH ABUTS AGAINST ILrUM. THE HEAD OF THE RIGHT FEMUR AND 

 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 in the National Museum the actual differences 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. Jouin. Sci., vol. 34, pt. 9, 1887, p. 41fi. 



