THE LOWER COAL-MEASURES. 185 
rocks. Some years afterwards two species were referred to this 
genus by M‘Coy* from the Mountain Limestone of Derbyshire and 
Northumberland respectively. The Illinois Acondylacanthus bears 
a close resemblance to Leptacanthus of Agassiz. Both genera 
are long in proportion to their width, and are laterally ornamented 
by numerous longitudinal strie. Their postero-lateral angles in 
each case bear a row of closely set recurved denticles. Hoplonchus 
differs much in all these respects ; it is a smaller spine, and in pro- 
portion to its length is broader, and more rapidly and gracefully 
tapers to a point; the striz are much less numerous, are better de- 
fined, and regular in position. Hoplonchus is further distinguished 
by the crenulations on the ganoid ridges, which do not occur in 
either of the other genera. 
Homacanthus is the name given by Agassiz to a small Ichthyo- 
dorulite from the Devonian formation of Russia. Its flanks are 
ornamented by longitudinal homogeneous furrows, and its posterior 
margins are armed with a double row of minute denticles. It 
is distinguished from Leptacanthus by the small number of the 
ridges, and the much greater breadth of the spine compared with its 
length. M. Agassiz, in the ‘ Poissons fossiles du vieux grés rouge,’ 
says, “‘the great difference which distinguishes them is, that the 
furrows or ridges of Homacanthus extend over the whole surface of 
the spine as far as the denticles of the posterior edge, whilst in those 
of Leptacanthus the ranges of teeth are preceded by a smooth space.” 
This character, however, does not hold good with all the species 
placed by Agassiz under the genus Leptacanthus. M‘Coy, who added 
two species, H. macrodus and H. miecrodus+, to the H. arcuatus 
of Agassiz, remarks that “the genus is by no means a well-defined 
one; but when confined to these three species, it has a sufficiently 
distinct facies.” The species figured by M‘Coy are both from the 
Mountain Limestone of Armagh. They are both imperfect. 
The smaller and decidedly curved fin-rays of Hoplonchus, which 
it is here suggested may have been attached to a posterior dorsal fin, 
certainly bear a great resemblance to the spines of Homacanthus ; 
and excepting that they are more gracefully proportioned, the small 
number of striz and the recurved sharply-pointed denticles might 
indicate that the genera were the same, and it is possible that future 
researches may necessitate the union of Homacanthus and Hoplonchus. 
CrENACANTHUS HQUISTRIATUS, sp. nov. (PI. X. fig. 15.) 
The length of this spine is nearly six inches. It is gently curved, 
the anterior margin being somewhat more so than the posterior one. 
The length of the latter is 3,%, inches from the line dividing the 
exposed part of the spine from the base, which, when living, was 
imbedded in the body of the fish ; the length of the anterior margin 
is 4,1, inches. The dividing line forms a curve at an angle of about 
45° to the length of the spine. The greatest width at the junction 
* ‘British Paleozoic Rocks and Fossils,’ by Sedgwick and M‘Coy, p. 6383, 
pl. iii. G. figs. 13-16. 
t Op. cit. 
