On the Fossil Fishes of the Carboniferous Limestone Series of Great Britain, 483 
Family : PETALODONTID AI, Newberry and Worthen. 
The genus Petalodus was instituted by Prof. Owen in 1840 (‘‘Odontography” p. 61, 
pl. 22, figs. 3, 4, 5,), and considered a sub-genus of Psammodus, the crown of which 
is produced into a median ridge and compressed, so that the ridge terminates the 
contour of the crown, like a trenchant edge. In the type specimen from the collec- 
tion of the late Sir Philip Egerton, the trenchant margin is slightly convex and finely 
serrated, the crown is invested with a thin layer of dense enamel with a smooth and 
shining surface, around the base of the crown the enamel is disposed in a series of 
concentric lines which extend lower down on the posterior than on the anterior 
surface of the tooth, and the enamel terminates on both sides in a line which is 
convex towards the base of the tooth, contrary to the terminal contour of the enamel 
in the compressed teeth of the sharks. The osseous basis of the tooth terminates 
in an expanded obtuse convex margin. This lamelliform tooth is bent slightly 
upon itself, so that a vertical section exhibits a slightly sigmoid flexure. 
M. Agassiz recognized the genus Petalodus of Owen, and transferred his Choma- 
lodus acuminatus to it along with several others (‘“ Poiss. Foss.,” Vo]. IIIL., p. 174, 
footnote,) which were not described, but which he considered should form a family 
separated from the Cestraciontes, because of the sharp cutting edge characteristic of 
the teeth. The species included are the following :— 
Petalodus acuminatus (Chomatodus). Yorkshire, Glasgow. 
» hastingsiz, Owen (type). Armagh. 
Ss psittacinus, Armagh. 
Bi leevissimus. . 
a rectus. 3 
2 radicans. - 
»  Marginalis. 3 
a sagittatus. - 
The Petalodus rectus, Ag., is figured in the “Geol. Report of Col. Portlock,” tab. 14, 
fig. 9, and in 1854 M‘Coy redefined the genus and described and figured the species 
named by Agassiz as follows :—- (“ Palzeoz. Foss.,” p. 635, et seq.) 
P. acuminatus, Ag. (+ P. rhombus, M‘Coy.) 
P. hastingsiz, Owen (+ P. levissimus, Ag.) 
P. psittacinus, Ag. 
P. rectus, Ag. (+ P. marginalis, Ag.) 
P. sagittatus, Ag. 
The remaining species named by Agassiz was elevated to form the type ofa new 
genus Polyrhizodus magnus (= Petalodus radicans, Ag.) characterized by the base, 
