190 FIELD COLUMBIAN MUSEUM. >» 
but is reduced is size, disappearing entirely with v. The infra-costal 
fosse begin with 11, become most pronounced in tv, and disappear 
with v. f 
The neural arch is low and massive throughout the caudal series, 
the neural canal forming a median groove in the upper side of the 
centra. The pedicles in the first caudal vertebre, in addition to being 
hollow, are excavated laterally by deep cavities, which enter them on 
the anterior side of the lateral plate. They give rise also to the thin, 
vertical plate which connects the diapophyses and the caudal ribs. In 
the second caudal one pedicle is hollow, the other solid. Caudals 
11 to v, inclusive, have the pedicles excavated at the base posterior to 
the lateral plate. From this point backward the neural arch becomes 
a simple, low, massive structure, giving rise to the zygapophyses and 
the neural spines. 
The neural spines are similar in their elementary structure to those 
of the posterior dorsal vertebrae. They are composed primarily of a 
stout median plate and a pair of slighter flanking plates. The median 
plate is rugose on the anterior and posterior margins for the attach- 
ment of inter-spinous ligaments. Near the crest the lateral plates are 
also thickened and rugose for muscular attachment. In the anterior 
members of the series the crest is cruciate in cross-section. ‘Two 
pairs of lateral buttresses which arise from the pre- and postzygapophy- 
ses and join the lateral plate, act as a set of brace-roots in resisting 
antero-posterior strains. These lateral buttresses become rapidly 
reduced as we pass backward in the series, and disappear entirely in 
x11. In the first caudal vertebra the spine is similar in height to that 
of the last sacral, and stands almost vertical, but the spines of the suc- 
ceeding vertebre are rapidly reduced in length and inclined backward 
(Plate tit). In xiv the spine is reduced to a short, blunt median 
plate and is but little expanded distally. With xvi the spines begin 
to elongate antero-posteriorly. 
The zsygapophyses are small in comparison with the size of the 
vertebre. They articulate in this specimen as far back as the twenty- 
third vertebra. The prezygapophyses are borne upon a pair of but- 
tresses, which spring from the anterior surface of the neural arch. In 
the anterior members of the series they are supported, in addition, by 
two pairs of posterior roots, which arise respectively from the base of 
the lateral spinous plate and from the diapophysis, as in the thoracic 
vertebre. With the reduction of the diapophysial element in the 
lateral plate, the third root of the prezygapophysis merges into the 
first; the. second root also disappears in caudal x, leaving only the 
inferior buttress. The articulating surfaces are convex, and through- 
