THE IGUANODON AND HYE^OSAURUS. 
299 
enclose between them the large circular sacral foramina. The inferior angles of the free 
extremities of the five posterior ribs expand and coalesce to form a band completing the 
sacral foramina without (Plate XXVI. //')• The thickest and strongest of these septa 
is the second, it is also inclined sliglitly backwards ; the second or third posterior ones 
have a tendency forwards. The corresponding compressed rib-like process of the 
first sacral is perhaps chiefly formed by the transverse process, and does not appear 
to have reached the band above-mentioned. The vertical septa extended as high as 
the base of the spinal ridge, and appear to have had a convex upper edge and a con- 
cave lower one. Curious bony buttresses (Plate XXVI. 3*)of a triangular form seem to 
have partly roofed over some of the sacral foramina ; these are most probably remains 
of a lateral expansion of the side of the spinal platform, at right angle hence to the 
vertical portion of the transverse process. These parts are, however, so much muti- 
lated that we must have more perfect specimens for examination ere many interest- 
ing points can be fully elucidated. The band above-mentioned is curved longitu- 
dinally in its anterior two-thirds, with a concavity looking downwards and outwards, 
the posterior part of the arc being twisted slightly from within outwards on its axis ; 
the portion contributed by the two posterior ribs is convex externally, passing into 
the concavity at a very obtuse angle. The bands of opposite sides are much more 
closely approximated in front than behind, but are most remote opposite the angle 
just mentioned. 
This instructive specimen of the sacrum also points out the true position of the 
Ilium (Plate XXVI. A), the form of which is well seen in the two detached examples in 
the Maidstone Iguanodon*. The slender anteriorly prolonged extremity, which is sup- 
posed in the Report on British Reptiles to be the posterior, is only an exaggerated 
condition of the short spine projecting forwards from the ilium in the smaller Lacertee. 
From the form and position of the head of the femur, I am inclined to think that no 
part of the surface of the acetabulum is present in this mutilated specimen ; it is per- 
haps fractured (A"") across the neck or contracted portion, beneath which it would 
expand to contribute to the formation of the acetabular fossa. 
The beautiful and interesting fragment of the sacrum of a Dinosaur, consisting of the 
four posterior vertebral bodies anchylosed together, Plate XXVII. figs. 2, 3, presents 
certain differences in the form of the centrum, which are perhaps due to age and sex ; 
but I am inclined with Dr. Mantell to regard it as probably belonging to the 
Hylseosaurus, which must have presented a nearly similar structure of the pelvis. 
The sacral fragment referred to the Hyleeosaurus by Professor Owen, cannot at 
present be found to institute the necessary coiiiparison with the present specimen. 
The age and size of the individual appear to have had no very obvious relation to the 
occurrence of anchylosis in the sacral column, cis we meet with examples of very 
dissimilar size both anchylosed and separate. There is the same disproportion be- 
tween the central and extreme elements of this chain as we have seen in the sacrum 
* Philosophical Transactions, 1848, Plate VIII. fig. 28. 
