MR. J. W. HULKE OR THE POLACANTHUS FOXII. 
655 
under surface of the centrum is cylindroid ; in the three foremost centra a shallow 
median groove indents it longitudinally. The piers of the neural arches rest each on 
two centra, and the sacral nerves escaped from the vertebral canal across the middle of 
a centrum except the last nerve, which passed out intervertebrally between the last 
sacral and the first caudal vertebra. A stout lower transverse process stands out from 
the side of the sacrum along the line of junction of each two centra, and doubtless the 
outer ends of these processes coalesced in the usual looplike manner : all have been 
broken off, and are now missing. The upper transverse processes which project from the 
crown of the arch are small and inconspicuous ; they form, with the expanded summit 
of a dwarfed spinous process, the support of a stout dermal armour to be presently 
described. Comparison of the following measurements of the sacral centra with those 
of the prsesacral centra already given will make the greater bulk of the former very 
apparent. 
Sacral centra. 
No. 1. 
No. 2. 
No. 3. 
No. 4. 
No. 5. 
Horizontal diameter at the articular ends . . 
■¥r 
87 
87 
87 
87 
,, ,, middle of centrum. 
70 
70 
70 
70f 
82 
Length of centrum.. 
67 
67 
67 
62 
58 
Post-sacral vertebra (Plate 72, figs. 1 , 2 : 
; Plate 73, figs. 1, 
2 ; Plate 75, figs. 3, 4).— 
The 13 now remaining are certainly not a 
continuous 
series- 
—many 
intermediate ones 
are missing; most are from the root of the tail. 
The first caudal, known to be such by its exactly fitting the last sacral, differs from 
this most obviously by the centrum bearing entirely its own arch. It and those centra, 
which by their great size are referable to the root of the tail close to the sacrum, bear 
no chevron mark. These centra are short and wide ; their articular ends are both 
gently concave, and they have a depressed heart-shaped contour. In one of the best 
preserved the horizontal diameter of the articular end is 100 millims., that of the 
anterior end being about 90 millims., whilst the vertical diameters of the same ends 
are 64 millims. and 61 millims., and the length of the centrum measured along its 
neural surface is 52 millims. At their middle these centra are much contracted. The 
transverse process stands off from the side of the centrum horizontally just below the 
line of the neurocentral suture. The arch is low; the spinous process has a strong back¬ 
ward slant; the post-zygapophyses are large and prominent. Other caudal vertebra 
smaller than those just described differ from them mainly in the presence of chevron 
marks and in the greater concavity of their articular ends. At least 13, probably 
about 17 or 18, of the vertebrae in the fore part of the tail had transverse processes; 
two smaller vertebrae, which by their very small size belonged to near the end of the 
tail, have a simple cylindroid figure; the processes have disappeared, and the arch is 
reduced to a tubular hoop (Plate 7 5, figs. 3, 4). 
* Rather less than others. 
t The numbers in this line, except the last, are approximate. 
