654 
MR. J. W. HULKB ON THE POLACANTHUS FOXII. 
not quite agree with these numbers; the vertebrae, foot-bones, and dermal spines are 
fewer, and I was not able to identify any portions of Ilium, Ischium, or Pubis. 
Vertebral column .—This is now represented by 11 prsesacral, five sacral, and 15 
postsacral or caudal vertebrae. 
The 11 prsesacral vertebrae comprise six disconnected and five anchylosed in a 
continuous series. 
Of the six disconnected vertebrae three are fairly complete (Plate 70, figs. 1 , 2 ). The 
form of the centrum is cylindroid; it is long relatively to its breadth, slightly con¬ 
stricted at its middle and expanded at its articular ends, which are plane or very 
slightly concave, the concavity of the posterior surface being most evident. The 
antero-posterior extent of the neurapophyses at their attachment to the centrum nearly 
equals the length of this latter. Their anterior margin rises nearly vertically from 
the centrum, wnilst their posterior margin has a strong forward slant. The spinous 
'processes of all the prsesacral vertebrse are broken off and missing. The transverse 
processes in this series show a double costal articulation. In the level of the crown 
of the arch is a large, conspicuous, capitular, costal facet borne jointly by the arch and 
root of transverse process. It is directed outwards, and against it in two instances 
the rib-head, of an expanded discoid form, still abuts. Above this, the process, 
slender and trihedral in cross-section, is prolonged outwards and upwards above the 
rib-neck. It bore at its free end, as is shown by detached pieces, an articular surface 
for the tubercle of the rib. The length of the vertebral centrum (represented in 
Plate 70, figs. 1 , 2 ) is 75 millims. ; the horizontal diameter at the articular ends is 
52 millims., and the vertical diameter here 51 millims.; and the horizontal diameter 
at the middle of the centrum is 35 millims. The double costal articulation places 
these six disconnected vertebrse in the front of the trunk. Between them and the five 
anchylosed vertebrse several are doubtless missing, since these last are demonstrated 
to belong to the loins by the anchylosis of the hindmost of the series to the first sacral 
centrum. 
The lumbar centra (Plate 71, fig. 1 ) have a more attenuate form than that of those 
referred to the front of the chest. Their lateral surfaces slope inwards and meet 
somewhat angularly below. This is very apparent in the second in the chain. The 
spinous and transverse processes of all are broken off and missing. 
The average length of the centrum in this series is 75 millims., the horizontal 
diameter at the middle is 33 millims., 30 millims., 30 millims., 34 millims.; and that 
of the articular ends is 40 millims., 37 millims., 36 millims., 50 millims. 
Sacrum (Plate 71, fig. 1).—This has the usual dinosaurian structure ; it consists of a 
chain of five anchylosed centra much larger than the slender lumbar centra. Accidental 
cross-sections made by fractures show the form of the centrum to be remarkably 
depressed (fig. 2) ; it is, however, not improbable that this great excess of the 
horizontal over the vertical diameter may have been increased by pressure. These 
diameters are in the second centrum 70 millims. and 25 millims. respectively. The 
