496 On the Fossil Fishes of the Carboniferous Limestone Series of Great Britaan. 
Petalodus grandis, Davis. 
GPL, Ip, it, 1.) 
Tooth, this is in several respects the largest and strongest member of the group. 
It is 2°5 inches across the crown. The depth along the central axis of the tooth 
from the tip of the crown to that of the base is 2inches. ‘The centre of the coronal 
surface is highly convex; on each side fhe convexity gives place to a propor- 
tionately deep concavity, the lateral tips of the crown are again raised and stand 
prominently forward. The convexity and concavity of the surface are reproduced in 
the crest and anterior margin ‘8 of an inch apart, bounding the coronal surface, and 
give to them sinuous outlines which converge to form a pointed extremity on each 
side. The junction of the anterior and posterior surfaces of the crown form a crest 
at an acute angle, with an undulating outline independently of the sinuous one 
already mentioned. The surface of the crown, where not worn or broken, is covered 
with enamel, pitted near the basal portion, and, in its higher part, striated and 
ductiferous. Where the internal portion is exposed by a broken surface, the bony 
structure with innumerable canaliculi is easily distinguishable. The anterior median 
ridge is broad, prominent, smooth, and devoid of inferior foldings; as already 
observed it follows the sinuous outline of the surface of the tooth. Beneath the 
ridge the anterior surface of the root gradually retreats and becomes, in conjunction 
with the posterior surface, somewhat thin and acuminate, from the lateral 
extremities the base of the tooth becomes gradually contracted and terminates in a 
widely rounded extremity. The base or root is simple and undivided. Its surface near 
the anterior ridge is longitudinally furrowed, but the furrows disappear lower down. 
and the base becomes even and smooth, thickest in the middle and thinning off on 
each side, and forming an acute angle with the opposite surface. The anterior face 
of a second specimen is exhibited, ‘The antero-coronal ridge is produced in front 
retreating on each side and much depressed at each lateral extremity, the surface 
is generally convex, the posterior border evenly circular. The centre of the crown 
is worn in front. From the anterior ridge the basal portion retreats slightly, 
forming an unbroken root. 
This magnificent specimen bears a close relationship to the species of the genus 
Polyrhizodus found in the same locality ; in every respect except the division of the 
base into several rootlets, the characters of the two genera are similar. The largest 
species of Petalodus hitherto described is P. destructor N. and W. (“Geol. Hl.,” 
Vol. IL, p. 35., pl. ii, figs. 1, 2), from the coal measure limestones of Ilinois ; it is 
smaller in size than the species now described, considerably longer in proportion 
to the breadth, and both the crown and lower portion of the base are much more 
acuminate than in Petalodus grandis. 
Formation and locality: Mountain Limestone, Armagh. 
Ex coll. Earl of Enniskillen. 
