16 



PROCEEDINGS OF THE NATIONAL MUSEUM 



VOL. 77 



cupped on their outer ends for close union with the ribs. In the 

 posterior part of the series, these processes become so fully fused 

 that the line of demarcation between the two elements can be dis- 

 tinguished with great difficulty, as shown in Figure 11. 



In describing Ankylosaurus^ Brown ^^ points out that " in the four 

 posterior dorsals preserved the ribs are firmly coossified to the ver- 

 tebra." Three such vertebrae are present in this specimen, but this 

 condition probably continues in those dorsals ankylosed with the 

 sacrals. In Panoplosaurus mirus all of the ribs are said to be free. 



The pleuropophyses gradually rise from the top of the mid part 

 of the centrum to a point well up on the side of the arch. In the 

 mid-dorsal region this facet is large and cupped. The neural canal 

 is large and vertically oval but diminishes slightly in size posteriorly. 



The anterior zygapophyses are much enlarged, trough-like, and 

 united. The posterior zygapophyses have flattened articular faces, 



Figure 11. — Posterior dorsal vertebra with coossified rib of Palaeoscincus 

 RUGosiDENS. TYPE. No. 11868, U.S.N.M. Posterior view. About one-fifth 

 natural size 



not rounded as in Ankylosawus, and are distinct one from the other. 

 The spines are relatively short, thin plates with transversely en- 

 larged extremities, this enlargement being mainly on the posterior 

 half. 



The last free dorsal is peculiar in having the posterior articular 

 face of the centrum smaller than the anterior, and also in having it 

 raised to a higher level, which gives the vertebra an inclined appear- 

 ance when viewed laterally. In the articulated backbone this would 

 serve to accentuate the curve of the sacro-lumbar region. The noto- 

 chordal projections, if they may be so designated, described in the 

 cervicals, persist throughout the dorsal series, gradually increasing 

 in size posteriorly. On the most posterior vertebra they occupy 

 fully one-half of the entire articular face of the centrum, 



IB Bull. Amer. Mus. Nat. Hist., vol. 24, 1908. p. 193. 



