658 
MR. J. W. HULKE ON THE POLACANTHTJS FOS1I. 
of the keeled scutes have the deep surface angularly excavated. (Plate 71, fig. 7. 
Plate 73, figs. 1-4.) 
The spined scutes are fewer than either of the other two forms. They are all 
asymmetrical. Their form is rudely triangular, the shorter are obtuse and the longer 
acute. (Plate 71, figs. 4-6. Plate 76, fig. 1.) 
Their base is very stout, its outline is a rhomboid. The blade projects in one of 
the most perfect, which, however, wants the tip, to 30 centims. beyond the base. 
The long diameter of the base of this scute is 21 centims. and the shorter 11 centims. 
One edge of the blade is relatively straight and the other is incurved. A similar 
difference in the direction of the borders is seen in the smaller scutes of this kind. 
When a spined scute is placed upon its base on a hat surface the slant of the blade is 
seen to be considerable ; one surface, which in this position is upper, is nearly plane or 
sinuous transversely, whilst the other surface is transversely convex. The plane or 
sin nous surface has its distal moiety deeply furrowed by vascular grooves. 
With regard to the distribution of the different forms of scutes we have to guide 
us : a, Mr. Fox’s impressions of the armour as he saw it first before it was disturbed 
and broken up—he says that from its relations to the other bones he thought it covered 
the loins as a continuous shield ; (3, inferences drawn from the scutes themselves ; y, 
the preservation of a few scutes in situ in two regions. 
The upper surface of the sacrum is still overlaid by a continuous flat scutal covering 
ornamented with tubercles, which dot it irregularly without definite grouping 
(Plate 71, fig. 3). I did not detect in it any marks of joints, and am therefore 
disposed to regard this as forming part of one large plate, which is certainly in 
its natural position. 
The fortunate recovery of the piece sketched in Plate 73, figs. 1-3, shows that an 
upper row of carinate angularly excavated scutes covered the neural, and a lower 
row of similar scutes embraced the haemal spines of the tail. One of these keeled 
hollowed scutes, which from its large size was probably situated at the root of the 
tail, is 21 centims. long, 12'5 centims. high, and the angular excavation of its base is 
4 centims. deep (Plate 73, fig. 4). Another from near the end of the tail is only 
2 '5 centims. long by 5 centims. across, and its keel is quite dwarfed (Plate 75, fig. 5 ). 
The upper and the lower row of these keeled scutes did not quite meet, but they left 
a lateral interval filled by a series of smaller flat scutes (Plate 73, fig. 1). Both forms 
with the diminution of the bulk of the vertebrae underwent a corresponding reduction, 
and they became towards the end of the tail small button or buckler-like studs, one of 
which is shown by Plate 75, fig. 6. Thus the whole tail was sheathed in armour. 
It has been already mentioned that the spined scutes are asymmetrical. This alone 
would make it most unlikely that they formed a median dorsal crest. That they were 
not so placed in the lumbo-sacral region is demonstrated by the preservation of flat 
scutes there in situ. It is not improbable that the spined scutes and the unexcavated 
carinate scutes encircled with tubercles were grouped in lateral rows. 
