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Mr. T. Stock on the Genus Tristychius. 183 
from each other. In a few cases the base of one cusp rises 
from the base of its neighbour without any distinct space in- 
tervening. The middle cusp of one large tooth shows at its 
base small depressed elliptical areas enclosed by folds of the 
ridges. 
It is rather singular that this elegant tooth should for so 
long have escaped detection ; nevertheless, so far as I have 
been able to discover, it has not been previously described. 
Tt bears a general resemblance, however, to the teeth figured 
by Giebel * as Hybodus carbonarius and Hybodus vicinalis ; 
and the evidence is in favour of their being allied. 
A very pretty little tooth found by Mr. M‘Leish in the 
Calciferous-Sandstone series near Bathgate, and given to me 
by Mr. Henderson for examination, shows (Pl. VII. fig. 11) 
some deviation from the teeth in the Hailes specimen. In it 
the middle cusp is nearly of the same width from the base to 
the apex, and the ridging and sulcation are rather more regu- 
lar; but these are differences which may be expected to occur 
in different individuals of the same species and in different 
positions in the mouth.. I do not think it is specifically 
distinct. 
I have figured (Pl. VII. fig. 12), from another specimen, a 
few teeth that lie almost undisturbed. The information de- 
rivable from the Hailes specimen is rather meagre as to those 
points connected with the buccal cavity upon which light might 
be expected to be thrown by the discovery of a tolerably good 
specimen. After a prolonged scrutiny of the material, I think 
the following statements may be considered as at any rate 
not widely removed from the truth. The greatest width of the 
aperture was from 2 to 3 inches, probably more. The man- 
dibular and palatal cartilages were curved, the curves being 
broadly elliptical or circular. The teeth were set in nume- 
rous compact rows, one row behind another, in regular (not, I 
think, alternate) order, which (in a second specimen) are seen 
to occupy a space of about an inch, measured in the direction 
of the axis. 
The Spines.—In this and the following specimens I have 
found the spines associated with the teeth in an identifiable 
condition. In this they number four ; and though all are frag- 
mentary, the shape and sculpture of the fragments are well pre- 
served. ‘I'here is not the slightest doubt that they perfectly 
resemble each other in size and contour, and that the ornamen- 
tation is identical in every respect. ‘There is also no doubt 
that they are easily identifiable with Tristychius arcuatus, 
* Loe. cit. Reproduced on Pl. VII. figs. 5, 5a, & 6,64, 
