400 On the Fossil Fishes of the Carboniferous Limestone Serves of Great Britain. 
This example, originally figured by Professor Agassiz as Psammodus subteres, and 
afterwards described as Helodus subteres, appears in some important characteristics 
to approach nearer to the teeth of Orodus than to those of Helodus. One of the 
most important distinctions between the two genera consists in the form of the 
base of the crown and its attachment to the root. In Helodus, the base of the 
crown spreads out from the central cone, more or less in every direction, but 
especially towards the anterior and posterior margins, forming in many instances 
quite a concave flexure from the apex to the basal margins. The apex of the crown 
is generally more or less acutely pointed or knife-like. The root of Helodus was 
attached to the palate in a superficial manner as compared with that of Orodus, and 
consists of a thin, deeply concave, often short, bony substance. On the other hand, 
the crown of Orodus descends from the median cone and lateral surfaces with a 
convex flexure, and the basal margins are incurved and rounded : the apex of the 
cone is rounded, generally more or less bulbous, and the root is long, comparatively 
thick and openly porous. 
The specimen described by Agassiz as Helodus subteres has a median cone 
which is obtusely rounded, the lateral prolongations, though not possessing the 
median ridge which the Orodi generally do, is rounded and the basal margins are 
incurved. There is also a portion of the root preserved which is decidedly Orodont 
in appearance. It is thought advisable, notwithstanding the smoothness of the 
surface to recognize the characters indicated and to transfer the specimen to the 
genus Orodus. 
Formation and locality : Mountain Limestone, Black Rock, Bristol. 
Ex coll. Bristol Museum. 
Genus—Petrodus, M‘Coy. 
Petrodus—MOoy, F., 1854. “ Brit. Paleoz. Foss.,” p. 637. 
“Teeth, conical, supported on a nearly circular osseous base, concave beneath ; 
crown with a dense compact surface, height not exceeding the width, deeply 
furrowed with rough radiating ridges.” —(M/‘Co7.) 
Petrodus patelliformis, M‘Coy. 
(Pl. LI, figs. 16, 16a, 160.) 
Petrodus patelliformis—F. M‘Coy, 1848. ‘Ann, and Mag. Nat. Hist.,” Second series, Vol.II., p. 132. 
J. Morris, 1854. ‘Cat. Brit. Foss.,” p. 337. 
1855. “Brit. Paleoz. Foss.,” p. 637, pl. 3G., figs. 6, 7, 8. 
2? 9) 
” 2? 2? 
Teeth, “conical, height one-half to two-thirds the width of the base, which is round 
or rarely subtrigonal ; apex rudely pointed, becoming flat by wear ; sides radiatingly 
ridged with about thirteen or fourteen very strong, single or dichotomous ridges, 
the sides of which are usually cut by numerous deep oblique sulci; the ridges are 
