MR. J. W. HULKS ON POLACANTHUS FOXJI. 
171 
areolated or cancellated arrangement of the bony tissue; and here may be seen 
numerous vascular canals surrounded by concentrically placed lamellae. 
Thus the shield doubtless in part represents the cutis vera. It must have been 
invested by a vascular membrane analogous to the periosteum of the endoskeletal 
bones, and externally this was overlaid by an epidermal covering which sheathed the 
keels and spines. 
Upon the inner surface of the shield are apparent in the middle line the sacrum and 
a series of lumbar vertebrae synostosed and forming a rigid bony rod (as described in 
1881). The lower transverse processes, imperfectly known to me in 1881, are now 
well shown. 
Those of the 2-5 sacral vertebrae average 14-15 centims. in length. Their 
stout, expanded, distal ends abut against the median surface of the ilia. The lower 
transverse processes of that which I have called the first sacial vertebra are less 
stout, and their direction differs slightly from that of the others ; their connection with 
the ilia is also less evident. It is, therefore, possible that this vertebra may better 
claim to be the last of the lumbar series. (In Iguanodon this vertebra is often found 
synostosed with the first sacral, and so functionally composing part of the sacrum.) 
Five ribs on the right side still retain nearly their normal relations with the lumbar 
vertebrse. They progressively shorten from before backwards, their lengths decreasing 
from 28 to 25 centims. ; these are approximate measurements, because the distal 
ends of the ribs merge into and are lost in the inner surface of the shield without 
distinct indication of their actual termination. In their vertebral halves these ribs 
exhibit well the inferior ridge which gives a triquetrous figure to their cross-sections, 
and must have greatly increased their strength. The vertebral ends of the ribs are 
crossed superiorly by longitudinally disposed bundles of slender bony rods. These are 
manifestly ossified tendinous and ligamentous structures, similar to those which I 
have mentioned as occurring in Hypsilophodon Foxii, and to those in Iguanodon lately 
described by Mr. Dollo. 
The pelvis, of which in 1881 little was recognisable, is now worked out. The 
acetabula are well displayed ; their large size immediately attracts the eye. The ilia 
are so blended with the shield that their exact form is not discernible. So far as 
slight textural differences of the surface warrant an inference, I am disposed to think 
that the prse-acetabular was longer than the post-acetabular portion, but I speak with 
reservation on this point. As in many Dinosaurian ilia, the pubic part of the 
acetabular arc of the ilium forms a strongly marked angle, from which abruptly rises 
the lower border of the prse-acetabular process. The remains of the os pubis are too 
fragmentary to give the shape of this bone, but 1 think that indications are 
recognisable of its division into a prce-pubic and a post-pubic part. Other specimens 
must, however, decide this. 
The ischium is better preserved, the left being nearly entire. It has a compressed 
doubly-curved figure of simple form, decreasing from a breadth of 1 5 centims., where it 
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