32 PERMO-CARBONIFEROUS VERTEBRATES FROM NEW MEXICO. 



The remaining presacrals are similar in form. The neural spines become a 

 little heavier and shorter, the neural arches are narrower, the transverse processes 

 become shorter by reduction of the lower end, not reaching to the anterior end of 

 the centrum, and the position of the neural arches changes so that the posterior 

 zygapophyses hardly reach beyond the posterior edge of the centrum, while the 

 anterior ones reach far forward, almost to the middle of the preceding centrum. 

 In the dorsal series the condition is almost exactly the reverse of this last-mentioned 

 point. Between the last two presacral vertebrae the intercentra are very broad 

 antero-posteriorly, but still quite shallow. 



The sacral vertehrcB are two in number ; both have lost the neural spines. The 

 first has a very short and narrow neural arch with well-developed anterior but 

 weak posterior zygapophyses. The sacral rib of the first is very heavy; its proxi- 

 mal attachment is from the anterior end of the anterior zygapophysis to beyond 

 the middle of the neural arch. The body of the rib extends nearly straight down- 

 ward, but has a slight posterior inclination. The lower end is concealed in large 

 part by the ilivun, but it can be seen that it is a broad, thin plate with a wide pos- 

 terior extension, which was applied to the inner side of the crest of the ilium. 

 The whole vertebra is set rather high, so that the crest of the ilium does not rise 

 greatly above the middle of the neural arch. 



The second sacral vertebra resembles the first in most regards, but the ribs are 

 very much smaller and more slender. Starting with a strong attachment to the 

 sides of the neural arch and the centra, they rapidly dwindle to a very slender 

 process without any distal expansion. The lower part extends downward and then 

 turns backward, just touching the inner side of the ilium. These ribs take very 

 little part in the support of the pelvis and resemble the ribs of the anterior caudal 

 vertebrae very closely. It might be said that there is but a single sacral vertebra, 

 so small is the share of the second sacral vertebra in the support of the ilium. The 

 sides and bottoms of the two vertebrae are hidden by matrix, but as they are slightly 

 separated in the specimen it may be said with certainty that they were not closely 

 united in life. 



The anterior caudal vertebra closely resembles the second sacral. The spine 

 is the only one of the caudal series which is preserved. It is short and sharply 

 curved to the rear, so that the flat distal end looks directly to the rear. The ribs 

 have strong triangular heads firmly attached to the centra and occupying most 

 of the side; they are rather elongate and, like the rib of the second sacral, extend 

 downward and outward, but turn sharply to the rear at the distal end. The lower 

 face of the centrum is wide, slightly concave antero-posteriorly, and gently convex 

 from side to side. There is no trace of a keel. 



The second to the sixth caudals have the same form as the first, becoming 

 gradually smaller. The ribs maintain the same form, but rapidly shorten. The 

 lower surface of the first five is concealed by matrix; the sixth and seventh are 

 poorly preserved and as yet not freed from the matrix. It is uncertain at what 

 point the chevrons began. 



The eighth caudal has a relatively small neural arch set well forward on the 

 centrum. The spine was slender and inclined slightly forward, but, in common 

 with all the caudals, except the first, has been destroyed. The base of the centrum 

 is shortened by the development of a large face on the anterior end for the accom- 

 modation of the head of a chevron bone. The posterior end is beveled by a similar 

 but smaller face. Between the eighth and the ninth centra is the base of a badly 

 injured chevron; this is the only one of the series of chevrons that has been pre- 

 served and it is so badly injured that little more than its presence can be noted. 



