THE CAMBRIDGE GREENSAND. _ 605 
tebree in other Dinosaurs, these centrums are of dissimilar forms, 
and are unlike the dorsal vertebree, which all have the visceral sur- 
face well rounded, as in Crocodiles. 
The first sacral (fig. 3,1) is 1,5; inch long along the neural canal, 
and about ;4, less along the visceral border. The base and sides are 
flattened, though the sides are moderately concave in length, and 
slightly convex from above downward, in which direction they con- 
verge. The base is defined by well-rounded shoulders, which merge 
into the sides ; it is slightly concave in the middle towards the two 
ends. The anterior articular surface is much the smaller of the two, 1s 
vertical, flattened, slightly concave from above downward, and sub- 
quadrate, being wider above than below. From the neural canal 
to the base is ;®, inch. The width of the centrum just below the 
neural canal is 1,2, inch, while the width at the base is about ~% 
inch. The neural canal is very large, deep, widens rapidly behind, 
and gives off the first pair of sacral nerves anterior to the posterior 
articular surface, where the transverse measurement is 1} inch. In 
front the neural canal is about 8, inch wide; posterior to the exca- 
vations for the sacral nerves the width is probably nearly double. 
The posterior articulation is angularly crescentic, being concave above 
in the line of the neural canal ; it is ;8, inch deep, 13 inch wide at 
the upper margins of the neural groove in the centrum, and about 
= inch wide at the base. The surface is flat, but marked with 
grooves, which radiate towards the sides and base. The length of 
the wall for attachment of the neural arch is less than an inch; it 1s 
compressed from side to side, and increases in width in front. ‘There 
is no indication of a rib to the ilium having originated from the 
centrum of this vertebra. 
In the second sacral (fig. 3, 2) the centrum attains its greatest width, 
and the neural canal acquires its largest size; but in form the vertebra 
is unlike the first, especially being depressed and much broader than 
long, with a rough parallelism between the convex visceral surface 
and the concave neural canal. The extreme length of the centrum 
is 143 inch; but it is somewhat less in the median line, since the 
posterior face is concave from side to side. ‘The base is flattened, 
but rounds into the sides, which slope obliquely outward, more 
rapidly towards the posterior than towards the anterior end. In 
front the flat articular face of the centrum is 58, inch deep; it is of 
suberescentic outline, extending superiorly up each side of the neural 
canal, where the horns are 3 inch wide. The posterior articulation 
is an arc of a large circle, as wide at the sides as in the middle, where 
it is ;5; inch deep. ‘The pedicles for the neural arch reach from the 
anterior face of the centrum backward ;; inch, and diverge outward ; 
they are } inch wide, and narrow posteriorly. They are bounded 
behind by the large canals for the sacral nerves, larger on the left 
side than on the right ; these canals are directed obliquely forward. 
Behind and chiefly below these grooves are the large, subcircular 
facets, # inch in diameter, for the sacral ribs, which look outward, 
backward, and somewhat upward, extending to the posterior articular 
surface of the centrum, and by their transverse extension making 
Q.J.G.8. No. 140. 27 
