XXIII : PLECTOGNATHI 
N 
to 
= 
Sub-Order 13. Plectognathi. 
Air-bladder without open duct. Opercular bones more or less 
reduced ; supraoccipital in contact with the frontals, separating 
the parietals; maxillary and praemaxillary bones often firmly 
united. Pectoral arch suspended from the skull. No ribs. 
‘Ventral fins thoracic and much reduced if present; the pelvic 
bones, if present, more or less completely co-ossified. Gill-open- 
ing much reduced. Body covered with more or less osseous 
scales, bony scutes, or spines, or naked. 
A highly aberrant group, closely connected with the Acan- 
thopterygii through the Acanthuridae, as pointed out long ago 
by Dareste.' The skeleton is often feebly ossified and the 
vertebrae much reduced in number, but the jaws, although short, 
are very strong, usually with large sectorial teeth which may be 
confluent into a beak; the post-temporal is short and simple, 
suturally united to the squamosal. These fishes have usually 
been arranged in three divisions: Sclerodermi, Ostracodermi, and 
Gymnodontes, but Regan,” whose classification 1s here followed, 
has shown that the latter include a type (7riodon) which, in 
spite of its beak-like teeth, is more nearly related to the Sclero- 
dermi, whilst the Ostracodermi have much more in common with 
the latter than with the Gymnodontes. It therefore appears 
best to admit only two divisions, the first with 4, the second 
with 3 families :— 
I. SctERODERMI.—Supraclavicle vertical ; pectoral arch of the Perciform 
type; all the vertebrae with a single neural spine. 
A. Body covered with hard or spinous scales; epipleurals present ; 
pelvis present. 
Teeth separate ; spinous dorsal present ; ventrals paired; pelvis immovable 
1. Triacanthidae. 
A beak ; spinous dorsal and ventrals absent ; pelvis movable 
2. Triodontidae. 
Teeth separate; spinous dorsal present ; ventrals absent or represented 
by a single short spine; pelvis movable : . 3. Balistidae. 
B. Body encased in a carapace; no epipleurals; spinous dorsal, pelvis, 
and ventrals absent : ; : . 4. Ostraciontidae. 
1 Ann. Sci. Nat. Zool. (3), xiv. 1850, p. 105, and C. R. Ac. Sct. Ixxiv. 1872, 
p: 1527. 
2 Proc. Zool. Soc. 1902, ii. p..284. 
VOL. VII 
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