52 



BULLETIN 89, UNITED STATES NATIONAL MUSEUM. 



a 



Nopcsa * concurs in this view when he says this upward inclination "is rather 

 due to the increasing weight of the double row of dorsal plates." To me the above 

 reasons do not seem to be the logical explanations for these modifications in the 

 stegosaurian skeleton, particularly since there is no contact between the plates and 

 the vertebrae, and also since there is a very similar elevation of the transverse 

 processes in Trachodon, Triceratops, and even in the Sauropod dinosaur Haplo- 

 canthosaurus, all of which are considered to be. unarmored animals. 



The zygapophyses are large and obliquely placed, those in front looking upward 

 and inward, those behind looking downward and outward, as is well shown in 



figures IS, 19, 20, and 21. There is 

 no zygosphene articulation of the 

 vertebrae in Stegosaurus. 



The zygapophysial articulating 

 surfaces are elongated ovals, the 

 longest diameter being trans- 

 verse. They remain fairly constant 

 in size throughout the series, but 

 narrow antero-posteriorly in the 

 posterior membeis of the vertebral 

 column. 



Dorso-Sacral (Presacral No. 

 27) . — The centrum of this vertebra 

 is short, and firmly cooesified with 

 both the vertebrae in front and 

 back. (s(?.,fig. 23.) Likewise the 

 spinous process is firmly cojoined 

 with those of the sacrals. 



This vertebra lies distinctly in 

 front of the neck of the ilium, and 

 from the fact that it does not bear a sacral rib, and does have a short, stout dorsal 

 rib, even though that rib is partially metamorphosed, it can very properly be con- 

 sidered the last dorsal (Z', fig. 22, and sd, fig. 23). 



The rib mentioned above is plainly shown on the right side of specimen No. 4934 

 (y, fig. 23) . The head is firmly coosified with the diapophysis, the shaft being directed 

 forward and outward to the ilium, with which, in conjunction with a strong upward 

 and forwardly directed process from the first sacral rib, it gives strong support. A 

 narrow transverse slit separates the rib from the outwardly directed bar of the first 

 sacral, although in aged individuals these borders would doubtless be joined. 



The transverse process, unlike those of the dorsals more anteriorly, is much 

 depressed, coming out nearly at right angles to the neural spine. 



Fig. 21.— Dorsal veetebka of Stegosaueus ungulatus Marsh. 

 i Nat. size, a, Side view, b, Front view, d, Diapofhysis; 

 n, neueal canal; p, paeapophysis; s, neural splke; z, anterior 



ZYGAPOPHYSIS; z' ' POSTERIOR ZYGAPOPHYSIS. AFTER MARSH. 



1 Geol. Mag., vol. 8, No. 3, 1911, pp. 112-113. 



