PLECTOGNATHI 721 



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 irnited to the squamosal. These fishes have usually 

 been arranged in three divisions : Sderoclermi, Ostracoclei'mi, and 

 Gymnodontes, but Eegan,^ whose classification is here followed, 

 has shown that the latter include a type (Triodon) 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. ScLERODERMl. — Supraclavicle vertical ; pectoral arch of the Perciforni 

 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 ; veutrals paired ; pelvis immovable 



1. Triacanthidae. 

 A beak ; spinous dorsal and ventrals absent ; pelvis movable 



2. Triodoniidae. 

 Teeth separate ; spinous dorsal present ; ventrals absent or represented 



by a singh; 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. S. Ac. Sci. Ix.xiv. 1872, 

 p. 1527. ^ 



2 Proc. Zool. Sac. 1902, ii. p. 284. 



VOL. VII 3 A 



