On the Fossil Fishes of the Carboniferous Limestone Series of Great Britain, 341 
base occupies nearly one-fourth the length of the spine. The spine is crushed and 
a part of the base has disappeared ; from the widest part, a little higher than the 
line dividing the base from the exposed part of the spine, it becomes rapidly 
narrower and ends in a fine point. The sides are divided into six or eight striz, 
not so broad as the intervening hollows; the striz are simple and coated with 
enamel, 
Locality : Armagh, Mountain Limestone. 
Ez coll. The Earl of Enniskillen. 
Ctenacanthus levis, Davis. 
(IPL, Ls Wop tke The) 
Fin spine, about five inches in length and at its broadest part ‘6 of an inch wide, 
From the oblique basal line it tapers with a gradually diminishing diameter to an 
obtuse point. The spine is gently but decidedly curved in outline : elliptical in trans- 
verse section. The lateral faces are much compressed, and are covered with longitu- 
dinal ridges, extending from the basal line parallel to the posterior margin towards 
the point. The ridges are most numerous near the base where the spine is broadest, 
and become fewer by running into, and being absorbed by, a median ridge stronger 
than the others and forming a keel along the anterior margin the whole length of 
the exposed part of the spine. ‘he ridges, twenty-four in number at the base, are 
wider than the intermediate furrows, and present a broad flattened surface of shining 
enamel. The relative size of the costee and intercostal spaces 1s maintained through 
the whole length of the spine, being equally wide near the apex as atthe base. The 
base of the spine is comparatively small, of the usual fibrous structure ; the line 
dividing it from the exposed portion is straight and forms a very obtuse angle to 
the longitudinal axis of the spine. A deep channel extends along the basal portion, 
which becomes enclosed and forms an internal cavity extending nearly to the apex 
of the spine. The cavity is oval in section and is situated nearer the posterior than 
‘the anterior surface, equidistant from the sides. Along the posterior surface there 
extends a deep circular groove, the postero-lateral angles of which are armed for 
some distance below the apex with recurved, rounded, sharply-pointed denticles 
separated by spaces about double their own diameter. Lower down, the angles are 
devoid of denticles. somewhat more rounded, and quite smooth. 
This form of spine is comparatively rare, the specimen of which a figure is given 
is imperfect, the apex being broken off and a portion of the base fractured. Other 
examples are slightly more curved than the spine described, but are otherwise in all 
respects characterized by the same peculiarities. The difference in convexity may 
be a little irregularity, and due to there having been a spine in front of each of the 
two dorsal fins, one having been before the anterior and the other the postericr 
dorsal fin. 
