CARBONIFEROUS FISH-FAUNA. 
opercular apparatus equalling nearly one-quarter of the total 
length of the fish and somewhat exceeding the maximum depth 
of the trunk; caudal pedicle slender, less than half as deep as 
the abdominal region. Distance between the origin of the paired 
fins equal to that between the origin of the anal and caudal fins ; 
dorsal completely in advance of anal fin. External ornament 
chiefly of strive; teeth stout. Principal scales of flank not 
deeper than broad, with straight upper and lower edges; outer 
face of all the scales completely ornamented with fine, somewhat 
oblique ridges, which terminate in the delicate serrations of the 
posterior border; the ridges on the flank-scales nearly parallel, 
though often bifureating and intercalated; those on the lower 
half of the narrow ventral scales curved upwards at the front 
border, and most of them terminating behind at the diagonal 
which joins the antero-superior and postero-inferior angles. 
General Form.—tThe type-specimen is distorted in the ventral 
part of the abdominal region and incomplete dorsally ; but the 
original outline of the fish would probably be almost as repre- 
sented in the diagrammatic sketch, Pl. LX, fig. 3. The tail is 
shown to be inequilobate, as usual in Hlonichthys. 
Head.—Although the form and proportions of the head are 
indicated in the type-specimen, its details are obscure and only in 
part traceable. The greater portion of the cranial roof is shown 
as an internal cast, with the mark of the longitudinal slime-canal 
traversing its frontal element; but a fragment of a_ plate 
posteriorly bears a finely rugose or striated ornament. The orbit 
was situated far forwards, as proved by an impression of two of 
the circumorbital plates. The outer face of the maxilla is not 
preserved, but its oral border is distinct, curving downwards at 
the hinder end and bearing impressions of very stout conical teeth 
(Pl. IX, fig. le). Its great postorbital expansion is about three 
times as long as deep. The slender mandible is very imperfect, 
but fragments of the bone in its hinder part seem to show an 
ornament of longitudinal striations. The characteristic narrow 
opercular apparatus is recognisable, with occasional traces of a 
striated ornament. The forwardly-curved preoperculum is 
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