54 



BULLETIN 89, UNITED STATES NATIONAL MUSEUM. 



presacral vertebrae and in one species at least (S. ungulatus) by the addition of an 

 anterior caudal. 



In No. 4934 the first coossified vertebra bears a free rib (see X, fig. 23) which was 

 found still articulated with it, and this vertebra can, therefore, be referred to the pre- 

 sacral series. The next one posteriorly is firmly cojoined to the sacrals by the neural 

 spine and the shortened rib has become firmly coalesced with the diapophyses. This 

 iimer end of the rib abuts against the ilium and unites with a process from the first 

 sacral rib, together forming a considerable support to the ilium (y, fig. 23) . Since this 



vertebra does not support 

 a sacral rib it may very 

 appropriately be called a 

 dorso-sacral. In the sa- 

 crum of this specimen 

 there remain but three 

 coossified elements that 

 may properly be consid- 

 ered as being true sacral 

 vertebrae. The last ver- 

 tebra, which corresponds 

 to the fourth sacral in S. 

 ungulatus (compare fig. 

 22 with fig. 23), has a 

 process which articulates 

 with the ilium, and is 

 regarded as a modified 

 caudal which will here be 

 called caudo-sacral. The 

 centrum is detached from 

 the third or last sacral 

 and the spine is also free. 

 The sacral centra, 

 like all others in the ver- 

 tebral column, are solid, 

 and they gradually in- 

 crease in size from front 

 to back. In S. ungulatus, 

 they are depressed verti- 

 cally, and transversely have broadly rounded ventral surfaces, without pronounced 

 ventral keel. In S. stenops, however, the centra are not depressed, and there is a 

 decided ventral keel. Each centrum supports its own parapophysis (sacral rib), 

 although there is a tendency to overlap the preceding centrum. 



The ribs are stout, with expanded proximal and distal ends. In adult specimens 

 the proximal end is firmly ankylosed with the centra and the distal ends coalesce 

 to form what has been called the sacricostal l yoke which articulates with the ilium. 



Fig. 22. — Sacrum and ilia of Stegosaurus ungulatus Marsh seen from below. 

 tV Nat. size, a, First sacral vertebra; ac, acetabular surface; 6, rib of first 

 sacral vertebra; e, rlb of last sacral vertebra; it, ilium; I, second dorsal 

 '-"".' sacrum or presacral no. 26 counting back from the skull; V, LAST 



DORSAL OR 27TH PRESACRAL; p t LAST SACRAL VERTEBRA. AFTER MAESH. 



i E. S. Riggs, Field Columbian Museum, Geol. Series, vol. 2, No. 4, 1903, p. 179. 



