36 



MEMOIRS OF THE CARNEGIE MUSEUM 



rugose, laterally expanded lamina probably homologous with the superior portion 

 of the superior blade of the diapophysial lamina of the dorsals. The zygapophyses 

 are small and have their anterior articular surfaces looking upward and inward, 

 while the posterior look downward and outward. The neural spines are simple, 

 directed upward and backward, and decrease in length rather rapidly, but gradually, 

 as we proceed posteriorly. The anterior spines are emarginate at the summit, and 

 this emargination is gradually accentuated, resulting in a cleft some five inches in 

 depth at the summit of the spine of the sixth caudal. Posteriorly to the sixth this 

 emargination becomes successively less pronounced and the spines of the tenth and 

 succeeding caudals terminate in rounded, somewhat expanded club-like rugosities. 

 All the caudal spines show numerous Avell-marked spinous cavities. In Nos. 84 and 



94 of our collections the caudal spines point 

 more decidedly backward than they are 

 figured in Osborn's paper. Caudals two 

 and three of No. 84 are coossified (patho- 

 logically) by their centra. In No. 94 cau- 

 dals seventeen and eighteen (?) are sim- 

 ilarly united, as shown in Fig. 11. The 

 principal vertebral characters mentioned 

 above are shown in Pis. III., IV., V., VI., 

 VII., VIII., and IX., and in the various 

 text figures. 



The Chevrons. — Commencing with the 

 second all the caudals bear intervertebral 

 chevrons except perhaps the small posterior 

 ones where they are probably wanting. 

 It was in reference to the peculiar shape of the posterior chevrons that Professor 

 Marsh proposed the name of Diplodocus [double raftered) for these animals. Marsh, 

 however, erroneously supposed all the chevrons borne by Diplodocus to belong to 

 this double-branched variety. We owe to Osborn the first description of the varied 

 forms assumed by the individual chevrons throughout the different caudal regions 

 of Diplodocus. From Cs. 2 to 6 inclusive the chevrons are nearly straight, com- 

 pletely surround the hcemal canal, below which the two arms are united into a long, 

 straight and laterally compressed spine. From C. 6 to C. 11 inclusive the chevrons 

 are less completely coalesced below the hsemal canal, and they are curved backward 

 and resemble somewhat the blade of a sickle. Commencing with C. 13 the chevrons 

 of this and the succeeding vertebrae assume the form described by Marsh and instead 



Fig. 11. Coossified caudal vertebrse of 

 Diplodocus carnegii, with chevron coossified. 

 About the 17th and 18th of No. 84. Seen from 

 the right side. One tenth natural size. 



