32 MEMOIRS OF THE CARNEGIE MUSEUM 



ossified throughout their entire length as in 84, but the spine of sacral three is quite 

 free from, though closely applied inferiorly to, that of the second sacral. This 

 would seem to indicate that the primitive Dinosaurian sacrum consisted of two 

 rather than three vertebr£e, a condition similar to that found in the Crocodilia and 

 most other living Reptilia. The fourth sacral in N-o. 94 bears a free spine and is 

 coossified by its centram with the third and does not differ in any essential respect 

 from that described by Osborn or from that which has been found to obtain in No. 

 84 of our collections. From the characters noticed above we may draw the follow- 

 ing conclusions. 



First. — That the primitive Dinosaurian sacrum consisted of not more than two 



vertebrse. 

 Second. — ^That in Diplodocus this primitive number of sacrals has been increased 

 to four by the successive modification of anterior caudals resulting in 

 the presence of three true sacrals with usually anchylosed spines and 

 a fourth less completely modified sacrocaudal supporting a perfectly 

 free neural spine. 

 Third. — That anterior to the true sacrals the last dorsal has been less modified 

 than the anterior caudals, but so changed as to function as a sacral 

 vertebra analogous to the so-called pelvic vertebra in Struthio. 

 The First, Second and Third, or True Sacrals. — As has already been noticed, these 

 have centra not only coossified with one another, but usually with those of the im- 

 mediately preceding and succeeding vertebrae also. The neural spines of sacrals one 

 and two are always coalesced with eacli other and usually with that of the third. 

 The centra increase in size posteriorly. Laterally they each support a pair of sacral 

 ribs which expand and coalesce distally to form a broad, thick plate of bone coex- 

 tensive with and closely applied to the acetabular portion of the ilium with which 

 it is united by suture. This forms a considerable portion of the inner and superior 

 portion of the acetabulum, as will be seen by a reference to Figs. 9 and 10. These 

 vertebrae are further united with the ilia through their diapophysial laminse, the 

 inferior blades of which are greatly expanded and unite inferiorly with the sacral 

 ribs and externally with the plates and crests of the ilia. They thus enclose on 

 either side two large sacral cavities which open inferiorly and superiorly. The 

 diapophysial laminae in the sacral vertebrae divide into two blades near the base of 

 the neural spines, one. anterior, the other posterior. These blades diverge in such 

 manner that the anterior blade of an}' vertebra abuts against, and coalesces with, the 

 posterior blade of the diapophysial lamina of the immediately preceding vertebra. 

 Thus the transverse process, as well as the inferior blade of each diapophysial lamina 



