Pisces. } UPPER PAL‘ZOZOIC VERTEBRATA. 635 
Genus. PETALODUS (Owen). 
Gen. Char.—Teeth transversely elongated, usually much compressed, thin, petal-shaped ; cutting edge 
serrated ; base of crown with several narrow, imbricating folds of ganoine, descending lower on the posterior 
than on the anterior face ; root large, oblong, thin, truncated below; lower edge obtuse, tumid. 
The dentine of the crown of these teeth is of extreme hardness, almost defying the lapidary, and having 
no comparison with the comparatively soft Psammodi, &c. Most of the longitudinal medullary canals 
extend to the surface only at the cutting edge, to which they give a fibrous character under the lens; the 
highly-polished surface of the crown being left without punctures, which however appear on the edge of the 
ganoine folds at the base of the crown. 
The genus is only carboniferous. 
PeTALODUS ACUMINATUS (Ag.) PI.3. G. fig. 4. 
Ref. and Syn.—Ag. Poiss. Foss. p. 174. t. 19. f. 11, 12, 13. + Petalodus rhombus M°Coy, Ann. Nat. Hist. 2nd 
Series, Vol. II. p. 125. 
. 
Desc.—Crown sharp, compressed ; anterior face of variable height, regularly rhombic, the upper and lower 
margins sometimes symmetrical, the lateral portions of each being nearly straight or slightly arched down- 
wards, and meeting in the middle at an obtuse angle, or more frequently the angle both of the apex and 
coronal ridge more or Jess rounded; the cutting edge is obscurely undulato-dentate in some specimens by short 
obsolete vertical furrows, and minutely crenulato-striated, more commonly without the vertical plicze, from the 
faint undulations of the edges; the lower margin is prominent and surrounded by five or six small imbrications 
of ganoine, these descend lower (as usual) on the posterior face, where also the mesial angulation is less ; 
surface smooth, with a few fine irregular longitudinal strize; root moderately large. Height of the middle of 
the crown six lines; width about one inch nine lines; depth of root about seven lines ; the proportional height 
of the crown being however variable, frequently greater than the above. 
I originally described the rare variety, in which the coronal folds are angulated in the middle so as to 
give a very wide, narrow, rhombic form to the crown, and haying the cutting edge longitudinally undulated, 
under the name of P. rhombus. A considerable number of specimens now before me incline me at present, 
however, to consider it a variety of the present species. 
Position and Locality.—Rare in the carboniferous limestone of Derbyshire; not uncommon in the dark 
earboniferous limestone of Lowick, and Belsey, Northumberland. 
Explanation of Figures.—P\. 3. G. fig. 4, inner aspect of tooth, natural size; fig. 4a, outer side of 
same tooth. 
Preratopus Hastnasu (Oven). 
Ref. and Syn.— Owen, Odon. t. 22. f. 3, 4. = Petalodus levissimus Ag. lists. 
Dese.—Crown very thin, scale-like, gently convex along the upper edge, turning abruptly downwards at 
the extremities ; base of the crown with a broad series of five or six imbricating folds of ganoine, towards 
which the crown slopes without convexity downwards and outwards so that they form the thickest and most 
prominent part of the tooth; they are arched downwards in the middle, but abruptly curved upwards, and 
downwards again at the lateral fourths of the length; root abruptly narrowed, flattened, tongue-shaped, one 
third deeper than the crown on the outer side. Surface highly polished, and smooth when perfect, the cutting 
edge alone being marked with a row of punctures (twelve in one line) ; when worn, however, a fine line extends 
from each of these punctures half way down the crown, producing a striation scarcely visible to the naked 
eye. Average width of crown, six lines; depth on the outer side two lines; depth of root, three lines ; width, 
four lines. 
