174 FIELD COLUMBIAN MUSEUM. 
slight; in vu it is shifted downward forming a median brace between 
the anterior and the inferior roots; in vill it appears as an anterior 
brace to the side of the inferior root; in 1x and x it descends quite to 
the anterior margin of the centrum and forms a second’ inferior sup- 
port equal in importance to the first. 
The tubercular facets are borne low on the extremities of the trans- 
verse processes. ‘They are irregularly concave and vary in size accord- 
ing to the strength of the ribs which they bear. In presacrals 1 to vili 
they face outward and slightly downward; in 1x and x their direction 
is aitered so as to face more downward than outward. 
The neural spines undergo a radical transformation in the dorsal 
region. (Plate xtv1.) The posterior members of the series represent 
the extreme development of the straight, median spine. From this 
point to the eleventh presacral, or, first cervical, they pass by a regular 
gradation from the simple to the bifurcate. This change is so regular 
that no point in the series can properly be designated as ‘‘nodal’’ but 
at the same time all are transitional. In presacrals 1, 11, and 111 the spines 
are similar in length to the anterior sacral. In 1v there is a noticeable 
shortening supplemented by a slight concavity on the anterior margin 
of the crest, which marks the first tendency toward bifurcation. In 
v and vi this concavity is deepened and the spine reduced in the latter 
to little more than half. the length of that in 11. At the same time it 
has increased in breadth and the lateral angles of the crest have become 
acute and somewhat produced. Each of the lateral plates give rise to 
two diagonal branches; the median plate is noticeably reduced. In 
vil the anterior aspect of the spine presents a broad, flattened surface. 
The median plate is reduced to a mere rugose ridge on the anterior 
surface; posteriorly it is somewhat more marked. The lateral angles 
of the crest are extended, forming a pair of lateral processes. sur- 
mounted by laterally flattened crests. However, the vestigial median 
plate still furnishes the chief anchorage for the dorsal muscles. In 
vill the median concavity descends to a level with the superior margin 
of the post-zygapophyses. The median plate is reduced toa mere 
roughening for muscular attachment which persists to the end of the 
series. Bifurcation may thus be regarded ascomplete. The spines in 
1x and x are slight and deeply excavated at their bases by latera 
vacuities. 
The fost-zygapophyses are supported inferiorly by a pair of but- 
tresses arising from the posterior surface of the neural arch, laterally 
by the, posterior roots of the transverse processes, and superiorly by a 
second pair of buttresses descending from the lateral plates of the 
neural spine. In the posterior dorsal vertebrz the superior buttresses 
