504 On the Fossil Fishes of the Carboniferous Limestone Series of Great Britain. 
back the surface rises and a sharp cutting edge is produced along the crest of the 
tooth. ‘The anterior median keel in this specimen is devoid of a ganoine fold and is. 
simply produced from the surface at a sharp angle. 
The difference in the form of the crown of these specimens may be accounted for 
if one of the teeth has been derived from the lower and the other from the upper 
jaw, the convex surface of the one would fit almost perfectly to the concave surface 
of the other. 
This species bears some resemblance to Polyrhizodus littoni, N. and W. (“ Geolo- 
gical Survey of Illinois,” Vol. II., p. 357; pl. iv., fig. 10). The American species is, 
however, less than half the size of P. elongatus, the anterior band or ridge is 
bow-shaped, with four closely approximated enamel folds, and there are only seven 
or eight radicles, which have not the same characters as those of the Armagh 
species. 
Locality : Rare in the Carboniferous Limestone of Armagh. 
#zx coll. Earl of Enniskillen. 
Polyrhizodus sinuosus, Davis. 
(HEM, 1h, dies, Ii, 12, 18) 
Teeth, considerable difference in size; largest examples 1:7 inch across the 
coronal surface, and 1'1 inch from crown to extremity of fangs; smallest size is 
‘4 across and °34 in height ; between the two a variety of intermediate sizes may 
be traced. Crown, flattened or slightly convex, compressed to a sharp edge along 
the crest; margins of the coronal surface unsymmetrical ; crest for the most part 
straight, bent at each extremity—on the right at a sharp angle, and on the left at 
a more obtuse one—to form the lateral outlines ; the anterior median ridge forms a 
sigmoidal curve, one-half the crown being double the diameter of the other. The 
greatest breadth is °5 of an inch near the centre of the tooth; to the right this is 
reduced by the tenth of an inch, to *25 of an inch, and becomes still more restricted 
towards its lateral extremity ; in the opposite direction the crown maintains its 
ereater breadth, ending laterally in an obtusely rounded margin. The ridge formed 
by the anterior margin is minutely corrugated transversely to its axis; the corrus 
gations extend a short distance over the surface of the crown. The posterior coronal 
surface is not exhibited. A section of one of the teeth shows it to have been deep 
and extended far towards the base of the root. The inter-basal space recedes consider- 
ably from the anterior margin of the crown, forming a deep hollow preceding the 
base of the rootlets. Its surface is smooth, or very slightly punctured, the lower 
part corrugated contiguously with the insertion of the roots. The root is deeply 
fissured, forming four large, prominent globose radicles of unequal size. They are 
1°35 inch across, being one-fifth less than the breadth of the crown. 
In the smaller specimens the radicles are longer and more attenuated in propor- 
tion to the size of the tooth. The general form of the tooth, the peculiar sigmoidal 
