THE CAMBRIDGE GREENSAND. 607 
ward. The neural canal again becomes narrower behind than it 
was in front. 
In the fifth sacral vertebra (fig. 3, 5) the centrum reverts very much 
to the form and proportions of an early dorsal, being compressed at the 
sides, having the base well rounded, and the margins of the articular 
surfaces well elevated. It is 14 inch long, and has the body of the ver- 
tebra relatively deep in proportion to the width. The anterior arti- 
culation is half an oval, with a prominent tubercle below the neural 
canal, and on each side of this tubercle is a depression ; the surface 
is otherwise flattened, but slightly convex from side to side; it is 
;°; inch deep in the middle, and about 152, inch wide in the upper 
third. The posterior articulation is more nearly subquadrate ; it is 
rather deeper, about as wide, is flattened, but concavely impressed 
in the median line below the neural canal. The facet for the sacral 
rib is small, and limited to the upper third of the side of the cen- 
trum. The neural canal has now become much smaller than in the 
first sacral, and the nerves are given off very high up, more than 
1,2; ich from the base of the posterior articulation, and hardly more 
than a quarter of an inch anterior to it. 
The sixth and last sacral centrum (fig. 3, 6) approximates in charac- 
ters to an early caudal ; but the neural canal is more deeply excavated 
than in the caudal region, while the large size of the attachment for 
the neural arch and the form of the anterior articulation show it to 
be sacral. It is 1,2; inch long in the line of the neural canal, and 
less in the basal measurement. The anterior articulation is less 
than an inch deep, and is widened transversely by the large facet 
for the sacral rib to about 1,2, inch, but the transverse measure- 
ment over those facets is 1,5, inch. Below the neural canal, which 
is only ;%; inch wide in front at the base, is a prominent tubercle, 
with a concayity on each side, as in the fifth vertebra, below which 
the surface is similarly marked with slight grooves, such as are usually 
seen in cartilaginous surfaces between which there is no motion. 
The posterior surface is more nearly circular, and measures more 
than an inch in depth and 1,3, inch wide ; in its centre is an elevated 
tubercle, as usual in caudal vertebre, and around this the surface 
is concave and marked with faint concentric lines. The base of the 
centrum is well rounded, but its sides are pinched in concavely below 
the facet for the last sacral rib; this facet appears to have been 
transyersely elongated and to haye looked outward. The attach- 
ment of the neural arch appears to have extended the whole length 
of the centrum, and to have been wider than in any of the other 
sacral vertebrae. 
This sacrum gives evidence of only four sacral ribs (Pl. XXXTY. 
fig. 4). The first, attached between the second and third centrums, 
and the second, between the third and fourth centrums, were massive, 
the former being 14 inch in diameter at its origin and the latter 
rather less. These were the true sacral elements, and would cor- 
respond to the sacrum of the Crocodile or Teleosaur. The third and 
fourth sacral ribs are small; the former is given off from the fifth, 
and the latter from the sixth sacral vertebra. The great enlargement 
272 
