126 PATAGONIAN EXPEDITIONS: PALAEONTOLOGY. 



rapidly. The first caudal has a large, broad and much depressed cen- 

 trum ; the neural arch is quite short antero-posteriorly, and very high 

 dorso-ventrally, but the canal is very small and the spine low ; the zyga- 

 pophyses are very largely developed, extending much in front of and behind 

 the centrum, especially the anterior pair, which are surmounted by promi- 

 nent, rugose metapophyses ; the transverse processes are immensely long, 

 extending across the full width of the posterior pelvic opening, and some- 

 what thickened at the ends, but not articulating with the ischia. The 

 second caudal is very similar to the first, but has a narrower, less de- 

 pressed centrum, and slightly shorter, decidedly more slender transverse 

 processes. In the third caudal the centrum is nearly as long as that of 

 the second, but is more cylindrical in shape ; the neural arch and zyga- 

 pophyses are somewhat reduced, and the transverse processes are consid- 

 erably shorter than those of the second vertebra ; at the distal end, they 

 are strongly decurved and have roughened faces for the attachment of the 

 first ring of the tail-sheath. The fourth vertebra has a still more slender 

 and cylindrical centrum, much smaller neural arch and zygapophyses and 

 shorter, though strongly decurved and expanded transverse processes. 



The fifth caudal displays a comparatively sudden diminution in the size 

 of all the processes, but the transverse processes are still large and de- 

 curved. In the sixth the neural arch is further reduced, and the trans- 

 verse processes are shorter, but are still prominent and decurved and 

 have broad external faces. In the seventh vertebra the transverse pro- 

 cesses are straight and very much smaller. The succeeding vertebrae 

 diminish rapidly in size, and the eleventh is very small ; in all of them 

 the centra are cylindrical, somewhat contracted in the middle, and the 

 various processes are rudimentary. 



Chevron-bones are well developed ; in the anterior region (Plate XXV, 

 fig. 8), these bones are long, slender and laterally compressed, with the 

 proximal end expanded and enclosing a triangular canal, and with the dis- 

 tal end furcate. Posteriorly the chevrons become shorter and wider and in 

 the middle part of the tail (fig. 9) they are rather short, thin plates, much 

 expanded in the antero-posterior dimension, broadened at the proximal 

 end and perforated by a small circular canal ; from the middle of the dis- 

 tal, or ventral border is given off a pair of prominent, curved processes, 

 one on each side. In the hinder region of the tail (fig. 10), the chevrons 

 are very small and canoe-shaped, resembling those of the armadillos. 



