318 ORDERS OF FISHES. 



side, and is bordered by long dorsal and anal fins. The bones 

 of the head are twisted in such a manner that both eyes are 

 brought to one side of the body. The fish keeps this side up- 

 permost, and is dark-coloured on this aspect ; whilst the oppo- 

 site side, on which it rests, is white. The mouth has the two 

 sides unequal, the pectorals are rarely of the same size, the 

 ventrals look like a continuation of the anal fin, and the bran- 

 chiostegal rays are six in number. The Pleuronectidce are only 

 known by two or three fossils, of which the oldest is the little 

 Rhombus minimus (fig. 280) of the Eocene deposits of Monte 

 Bolca, 



SUB-ORDER C. ACANTHOPTERI. This sub-order is character- 

 ised by the fact that one or more of the first rays in the fins are 

 in the form of true, unjointed, inflexible, " spiny " rays. The 

 exoskeleton consists, as a rule, of ctenoid scales. The ventral 

 fins are generally beneath or in advance of the pectorals, and 

 the duct of the swim-bladder is invariably obliterated. 



The chief living families of this sub-order are the Perch 

 family (Percidce), the Mullets (Mugilidce), the Mackerel family 

 (Scomberida), the Gurnards (Sclerogenida), the Blennies (Blen- 

 niidte), the Gobies (Gobiidce), and the Chaetodons (Chatodon- 

 tidce). The fossil representatives of this sub-order are mainly 

 Tertiary; but the genus Beryx (fig. 279) dates from the Cre- 

 taceous period. In the Eocene Tertiary of Monte Bolca occur 

 several remarkable forms, of which one of the most singular is 

 the Chaetodont genus Platax (fig. 281). 



SUB-ORDER D. PLECTOGNATHI. This sub-order is charac- 

 terised by the fact that the maxillary and premaxillary bones 

 are immovably connected on each side of the jaw. The endo- 

 skeleton is only partially ossified, and the vertebral column 

 often remains permanently cartilaginous. The exoskeleton is 

 in the form of ganoid plates, scales, or spines. The ventral 

 fins are generally wanting, and the air-bladder is destitute of 

 a duct. 



This sub-order includes the living Trunk-fishes (Oslracion- 

 tida), File-fishes (Balistida), and Globe-fishes (Gymnodontida), 

 The fossil forms are few in number, and the earliest date from 

 the Eocene Tertiary. They are chiefly noticeable for the re- 

 semblance to the true Ganoid fishes, produced by their parti- 

 ally ossified endoskeleton and by their possession of ganoid 

 scales. 



SUB-ORDER E. LOPHOBRANCHII. This is a small and unim- 

 portant group, mainly characterised by the peculiar structure 

 of the gills, which are arranged in little tufts upon the branchial 

 arches, instead of the comb-like plates of the typical bony 



