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STRUCTURE AND RELATIONSHIPS OF OPISTHOCELIAN DINOSAURS. 189 
above the middle, in the form of a stout process terminating in an 
expanded and rugose extremity analogous to the parapophyses. The 
diapophysial element arises from the lateral surface of the neural arch 
by anterior, posterior, and inferior roots, analogous to those of the 
diapophyses in the thoracic vertebre. The two elements are con- 
nected by a vertical plate, which bears on its lateral margin a thick- 
ened and rugose surface facing laterally. Passing backward ‘in the 
series the diapophysial element is reduced to a roughened, vertical 
ridge on the centrum in caudal iv. The parapophysial element per- 
sists as a stout, vertically compressed process as far back as Ix, disap- 
pearing entirely with x11. 
The centrum in the first caudal vertebra, as before described, is 
concave in its upper half, but convex below. The posterior face is 
irregularly convex. The interior of the centrum has numerous small 
cavities, especially at the rim of the articular ends. There are no 
well developed lateral fossz. With the second vertebra the typical 
proccelous form of centrum begins. The anterior concavity continues 
quite marked as far back in the series as the ninth; from this point it 
diminishes in prominence, disappearing entirely with the sixteenth. 
The posterior surface of the centrum is but slightly convex in any of 
the caudal vertebre. In the second caudal the surface is in general 
convex with a slight concavity a short distance above the center. 
Passing backward in the series this concavity increases in area to the 
obliteration of the general convexity, so that in the seventh the former 
predominates. From this point backward the posterior concavity 
persists, being most prominent from the tenth to the sixteenth. From 
this it will be seen that the proceelous type prevails from the first to 
the ninth and grades into an uncertain amphiccelous type which pre- 
vails as far back as the sixteenth, where it in turn gives place to an 
irregular amphiplatyan form. 
Two sets of cavities occur in the centra of the anterior caudal 
vertebre, the first above and the second below the root of the sacral 
ribs. These will be designated respectively as supra- and infra-costal 
fosse, according as they occur above or below the root of the caudal 
rib. These cavities cannot be regarded as constant characteristics, 
as they are sometimes present on one side and absent on the other. 
The lateral cavities in the centra persist as far back as caudal v in 
this specimen. In caudal 1 the supra-costal cavity is represented by a 
slight infolding of the periphery of the centrum on the anterior side 
of the lateral plate. In caudal 1 cavities are present on both the 
anterior and the posterior side of the lateral plate. In 11 there isa 
‘single supra-costal cavity on the posterior side; in Iv the same persists, 
