342 On the Fossil Fishes of the Carboniferous Inmestone Series of Great Britain. 
A transverse section of this species gives a triangle whose sides are connected by 
a base much shorter, and in this respect it is distinguished from Ctenacanthus 
plicatus, next described, in which the section forms an almost equilateral triangle. 
Locality ; Mountain Limestone of Armagh, 
Hx coll, Kar] of Enniskillen. 
Ctenacanthus plicatus, Agass. 
(Pl. XLV., fig. 4.) 
Onchus plicatus—L. Agassiz, 1833. “* Rech. sur les Poiss. Foss.,’’ Vol. III., p. 177 (ind). 
PA m J. E. Portlock, 1843. ‘ Rept. on Geol. Fermanagh, W&c.,” p. 461. 
5 5 C. G. Giebel, 1848. ‘ Fauna der Vorwelt,” Vol. I., pt. 3, p. 302. 
7 % H. G. Bronn, 1848. ‘‘ Nomencl. Paleont.,” p. 843. 
3 a . 1849. “ Knumerator Paleont.,” p. 652. 
S 55 F. J. Pictet, 1854. ‘“ Traité de Paléont.,” Vol. II., p. 284. 
. % J. Morris, 1854. ‘Catal. Brit. Foss.,” p. 334. 
ie A Morrisand Roberts, 1862. <‘‘Quart. Jour. Geol. Soc.,” Vol. XVIII, p. 101. 
5 J.J. Bigsby, 1878. ‘Thesaur. Devon.-Carb.,” p. 359. 
Spine, medium size, slightly curved, rather rapidly tapering towards the point, 
imperfect; length preserved three inches, probable length when perfect about 4°5 
to 5 inches, greatest diameter at line of insertion ‘65 of an inch ; base, osseous, 
striated, one-fourth the entire length of the spine: line dividing base and exposed 
surface, oblique, with convexity towards the base. Transverse section, triangular, 
sides acuminate above, base curved inwards, two-thirds the length of one side. 
Lateral surface covered by a number of broad plicated ridges, at 1:7 inches from 
the base, nine or ten in number, increasing by dichotomy to more than double that 
number towards the base, and apparently decreasing upwards. A peculiarity in 
the ridges of this species consists in their first dichotomizing, and then the two: 
branches lower down coalescing with the ridges on either side of them, to become 
again separated a quarter of an inch nearer the base. A representation of this 
peculiarity is given on Plate XLV., fig. 4a. The sides are very flat, meeting anteriorly 
ata sharp angle without median keel, the anterior margin being formed by the 
repeated inosculation of the lateral ridges. The latter present a steep, almost 
perpendicular face towards the anterior margin, whilst in the opposite direction 
they slope gradually downwards, presenting something like the appearance of 
overlapping tiles; a series of minute striations traverse the ridges in an oblique line 
parallel with that of the base. Posterior surface hidden by the matrix. 
This spine was named by Prof. Agassiz, Onchus plicatus, and is the type specimen 
from the late Admiral Jones’ collection. he specimen whichis described above as 
Ctenacanthus levis, was also named in MS. as belonging to the same species, in 
the collection of the Earl of Enniskillen. The principal characteristic given by 
Prof. Agassiz to distinguish Ctenacanthus from Onchus is that it is possessed of 
posterior denticles, the latero-posterior angles of Onchus being smooth and free from 
