34 MEMOIRS OF THE CARNEGIE MUSEUM 



The inferior blade of the diapophysial lamina has an extensive union with the crest 

 and posterior margin of the ilium. It encloses posteriorly a third and larger sacral 

 cavity which, like the two enclosed by the true sacrals, is left open superiorly and 

 inferiorly. 



In Fig. 9 is shown an inferior view of the sacrum and ilia found with skeleton 

 94, In this skeleton all the pelvic elements are complete, including the ilia, ischia 

 and pubes. The sacrum is also complete save the first sacral, which is represented 

 only by the centrum and sacral ribs. These are well preserved, but the neural 

 spine, neural arch and diapophysial laminse of this vertebra are for the most part 

 wanting. The pelvic vertebra is entirely missing, having evidently become detached 

 and removed from its normal position prior to the imbedding of the bones in the 

 matrix. The fact that this vertebra could have been detached and separated with 

 so little injury to the adjacent pelvic bones is in itself evidence of the imperfect 

 union between it and those bones through the medium of the centrum and of sacral 

 ribs. A comparison of Fig. 9 with that given by Marsh of the pelvis of Diplodocus 

 in his "Dinosaurs of North America," and reproduced here in Fig. 10, will show 

 that while that author was right in considering three as the number of true sacrals 

 in Diplodocus, he mistook the anterior for the posterior end of the sacrum, since our 

 material abundantly proves that the posterior sacral is the larger and the anterior 

 the smaller of the series, instead of vice versa as Marsh supposed. The apparent 

 similarity in the sacral ribs of these vertebrae in the two figures is due to the altered 

 view necessitated in the adjustment of the posterior end of the sacrum to the anterior 

 end of the ilium in the figure given by Marsh. His material having been found 

 isolated it was quite natural for Marsh to assume that the sacral with the larger cen- 

 trum was the anterior, but the reverse has proved to be the case. In Plate IX. may 

 be seen posterior and lateral views of the pelvis of No. 84 and a side view of a pelvis 

 of BrontosauTus for comparison. 



The Caudals. — As in a previous paper, Osborn's interpretation of the sacrals will 

 be accepted also in this paper. The number of sacrals is thus here placed at four, 

 while the caudals begin with the first vertebra posterior to that modified as a sacral. 

 The twelve anterior caudals are represented and for the most part are in an excellent 

 state of preservation in No. 84, Avhile associated with No. 94 there were found be- 

 tween twenty and thirty other caudals and several chevrons. These Avere for the 

 most part found disarticulated, and they doubtless pertain to two or more individ- 

 uals. The excellent caudal series belonging to the American Museum of Natural 

 History furnishes much the most trustworthy evidence regarding the number, - 

 nature and structure of the caudals in Diplodocus. After a careful study of this 



