346 On the Fossil Fishes of the Carboniferous Limestone Series of Great Britain. 
The type of this species is in the museum of the Geological Society, London, in 
the collection presented by Admiral Jones. It is an imperfect spine, 1°25 inches in 
length, -2 inch broad, and tapering toa point. The basal portion is wanting, and 
the specimen appears to be the superior extremity of a specimen perhaps three 
or four inches long when perfect. Its sides are ornamented by smooth, longi- 
tudinal ridges; a larger one forming a carina along the anterior margin. Be- 
tween the ridges are spaces wider than the ridges, in the flat bases of which are 
a number of minute striz running parallel with the ridges. The spine is enveloped 
in the matrix, and the form of a section 1s not perceptible. 
This little specimen appears to be unique. It was apparently considered by 
Prof. Agassiz to be an Onchus, and was named by that learned ichthyologist 
O. rectus. There is, however, no visible characteristic which will distinguish it 
from Ctenacanthus ; and as in all probability posterior denticles exist, but are 
hidden by the matrix, it is thought better to include the specimen in this genus. 
Formation and locality : Mountain Limestone, Armagh. 
Le coll. Geological Society’s Museum, London. 
Genus—Acondylacanthus, St. John and Worthen. 
Leptacanthus—L, Agassiz, 1833. <‘Poiss. Foss.,” p. 176. MS., (partim.) 
a EF. M‘Coy, 1855. ‘ Brit. Paleoz. Fossils,” p. 633. 
Ctenacanthus (partim)—F. M‘Coy, 1855. “ Brit. Paleeoz. Foss.,” p. 625. 
Leptacanthus—Newberry and Worthen, 1866. “ Paleont. Illinois,” Vol. IL, p. 116. 
Acondylacanthus—St. John and Worthen, 1875. “ Palzont. Illinois,” Vol. VI., p. 432. 
« Fin-rays, long, gradually tapering, lateraily compressed, and moderately curved 
posteriorly. Lateral faces longitudinally fluted, the costs being smooth (or 
crenulated) and enamelled, increasing by occasional bifurcation, perhaps more rarely 
by implantation. Posterior face uniformly excavated longitudinally, apparently 
without median keel; postero-lateral angles bearing a row of downward-hooked 
denticles, which extend in the majority of species well towards the base. Pulp cavity 
occupying the posterior half or more of the spine. Base unknown.’—(S¢. J. and W.) 
Prof. Agassiz, in his work “ Poissons Fossiles,” Vol. IIL, p. 27, has instituted the 
genus Leptacanthus, comprising several species of Ichthyodorulites from the Jurassic 
formation, including the Lias. They are long and slender spines; sides longitu- 
dinally striated, latero-posterior margins armed with teeth, and the anterior edge 
sharp and keeled; base not known, but probably like Hybodus. 
In the same work, page 176, Prof. Agassiz gives the name Leptacanthus priscus 
to a spine from the Carboniferous Limestone of Armagh, without observation. 
In the “ British Palzeozoic Fossils,’ p. 633, Prof. M‘Coy places two species, viz.:— 
L. junceus and L. jenkinsoni in the genus Leptacanthus, observing that the genus 
is a Carboniferous and Oolitic one. 
Messrs. Newberry and Worthen, in 1866, described a spine which was doubtfully 
