370 On the Fossil Fishes of the Carboniferous Limestone Series of Great Britain. 
Physonemus hamatus. 
(Pl. XLVIL, figs. 9, 11.) 
Onchus hamatus—L, Agassiz 1837. “Rech. sur les Poiss. Foss,” Vol. III, p. 9, pl. 1, 
figs. 7, 8. 
5 ©. G. Giebel, 1848. “Fauna der Vorwelt,” Vol. L., pt. 3, p. 302. 
3 5 H. G. Bronn, 1848. ‘ Nomencl. Paleont.,” p. 843. 
9 45 ns 1849. “‘Enumerator Palont.,” p. 652. 
¥ Fe J. Morris, 1854. “Catal. Brit. Foss.,” p. 334. 
95 % F, J. Pictet, 1854. “Traité de Paléont.,” Vol. IL, p. 284. 
3 ‘ Morris, & Roberts, 1862, ‘Quart. Journ. Geol. Soc.” Vol. XVIII, p. 101. 
5) , J.J. Bigsby, 1878. “Thesaurus, Devonico-Carb.,” p. 359. 
Prof. Agassiz described this form as a species of Onchus in the following terms : 
“Tt is distinguished from all other Ichthyodorulites by its strong curvature, which 
forms almost a semicircle. It rapidly tapers towards the point ; the base is simply 
but strongly indented and the internal cavity does not appear to extend the whole 
length of the spine . . . . . The surface is finely striated and the strie are 
smooth.” 
The genus Onchus is described as composed of species resembling the Lias Hybo- 
donts in all respects except that the posterior denticles are absent and the species 
are generally small in size. One species (O. hametus) differs from the remainder 
in its peculiar form and want of ornamentation, and Prof. Agassiz, whilst leaving it 
provisionally in the genus Onchus, expresses an opinion that it may be necessary to 
constitute a new genus for its accommodation. 
Though this species differs in some respects from the type in Prof. M‘Coy’s defi- 
nition of the genus, notably in its want of surface ornament, its general form so 
nearly approaches to Physonemus that it may be safely transferred from the genus 
Onchus to occupy a position so much more congenial. 
Several specimens of this species occur in the collection of the Earl of Enniskillen, 
and also in the Museum at Bristol. They are all from the Black Rock limestone of 
Bristol whilst all the specimens of Physonemus arcuatus and P. subteres have 
been found in the hmestone of Armagh. 
Formation and locality : Mountain Limestone, Bristol. 
Ex coll, Karl of Enniskillen. 
Genus—Chalazacanthus, Davis. 
Spine, medium size, strong, curved, tapering to a point, laterally compressed. 
Base large, striated, open posterior cavity, which higher up is enclosed. Base 
separated from the exposed portion by an oblique line. Surface of upper part 
ornamented by tubercles devoid of definite arrangement. 
This spine bears some resemblance to those of the genus Drepanacanthus 
N.and W., and Xystracanthus, Leidy (‘‘ Proc. Acad. Nat. Science, Phila 1859), but 
