348 On the Fossil Fishes of the Carboniferous Limestone Series of Great Britain. 
canthus distans, in the cabinets of Lord Enniskillen, but a comparison with the 
figure of that spine given on Plate XLVL.,, fig. 5, will prove sufficiently conclusively, 
that the two forms are quite distinct. A. colei bears a nearer resemblance to 
A. equicostatus, St. John and Worthen (Geol. Illinois,” Vol. VL., p. 634, pl. xvi., figs. 
12, 13), but it differs from that species in its more robust aspect, in the roundness 
of its section, and, so far as can be ascertained—for the posterior portion of the 
spine is slightly crushed—in the greater width of the posterior groove. 
I have taken the liberty to dedicate this species, which I regard as the type of 
the genus amongst British species, to the Earl of Enniskillen, whose magnificent 
collection forms the basis on which this memoir is built. 
Formation and locality : Mountain Limestone, Armagh. 
Hex coll, Earl of Enniskillen. 
Acondylacanthus tuberculatus, Davis. 
(Pl. XLVI., fig. 4.) 
Spine, imperfect ; the point and base not present. Remaining portion 2°5 inches 
in length, diameter nearest basal end °35 inch, gradually tapering to a point. 
Spine when perfect probably four inches in length. It is slightly curved, wore 
so on the anterior than the posterior surface. Transverse section of spine is 
triangularly elliptical, widest posteriorly, and somewhat accuminate anteriorly. 
Lateral surfaces compressed, covered with smooth enameled cost, about twelve 
in number near the basal extremity, and decreasing to five or six near the apex. 
The intermediate hollows are half the diameter of the costze, deeper than wide. 
The anterior surface is marked by a median keel, consisting of a similar ridge to 
those on sides, but larger and slightly broader. The ridge is coated with glistening 
enamel, produced to form a series of small tuberculose prominences, separated from 
each other by about their own diameter ; near the apex of the spine these tubercles 
become considerably smaller in size, more closely approximated, and somewhat 
irregular in disposition. The posterior surface channeled in basal portion, the 
cavity becoming enclosed, and continuing to within a short distance of the end. 
Each latero-posterior angle is armed with a row of acutely pointed, round, re- 
curved denticles, springing from a prominent and continuous ridge of ganoine 
which extends along and forms the angle independently of the lateral coste. The 
base not known. 
The species most nearly related to this one appear to be Ctenacanthus (2) 
burlingtonensis and C. keokuk, derived respectively from the limestones indicated 
by their specific names at Iowa and Illinois, and described by Messrs. St. John and 
Worthen in the “Palzeontology of Illinois” (Vol. VI., pp. 426, 427, pl. xv.,figs. 6, 7, 8). 
The two approximate very closely in specific characters and, according to the 
authors named, where a considerable series are together, it is not without difficulty 
