Families of Tdeostean Fishes. 189 



III. Gill-opeuing above axil of pectoral ; mouth rather small, sub- 

 terminal or inferior; pectoral fin strongly geiiiculated ; venti'.ils 

 present; spinous dorsal abs3nt or reduced to a small tentacle 

 lodged in a cavity under the snout 5, Malthida. 



Suborder XIIT. Plectognatiii. 



Air-bladder without open duct. Opercular bones more or 

 less reduced; supraoccipital in contact vvitli the frontals, 

 separating the parietals; maxillary and pncraaxillary bones 

 often firmly united. Pectoral arch suspended from the skull; 

 no mesocoracoid. No ribs. Ventral fins thoracic and much 

 reduced if present ; the pelvic bones, if present, more or less 

 completely co-ossified. Gill-opening much reduced. Body 

 covered with more or less osseous scales^ bony scutes, or 

 spines, or naked. 



A highly aberrant group, closely connected with t!ie 

 Acanthopterygii through the Acanthuridge, as pointed out 

 long ago by Dareste *. The skeleton is often feebly ossified 

 and the vertebroe 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 divi- 

 sions: Sclerodermi, Ostracodermi, and Gymnodontes; but 

 llegan +, 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 Sclerodermi, 

 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 four, the 

 f-:econd with three families : — 



I. Sclerodermi. Supraclavicle vertical ; pectoral arch of the Perci- 

 form type"; all the vertebra3 with a single neural spine. 



A. Body covered with hard or spinous scales; epipleurals present; 



pelvis present. 



Teeth separate ; si)inous dorsal present ; ventrals 



paired ; pelvis immovable 1. Triacanthidcc. 



A beak ; spinous dorsal and ventrals absent ; pelvis 



movable 2. Triodontidcc. 



Teeth separate ; spinous dorsal present ; ventrals 



absent or represented by a single short spine ; 



pelvis movable 3. Balistldcc. 



B. Body encased in a carapace ; no epipleurals ; spinous dorsal, 



pelvis, and veutrah absent 4. Ostraciontidce. 



* Ann. Sci. Nat., Zool. (3) xiv. ISoO, p. 105, and C. II. Ac. Sci. 

 Ixxiv. 1872, p. 1527. 



t r. Z. S. \W-2, ii. p. 231. 



