PERCIFORMES 437 



pelvic fins in the form of two large spines and a few jointed dermal rajs. 

 The scales are rounded and spiny. 



Family Triacasthidae. The premaxilla is protractile, and remains 

 separate. The hyopalatine arch is firmly fixed. From three to six dorsal 

 spines. The deep-sea genus Halimochirurgus has a long tubular snout. 



Acanthopleurus, Ag., Oligocene ; tyinacanthus, Ag., Eocene — Europe. 

 Triacanthus, Cuv. (Fig. 447) ; Triacanflwdes, Blkr. ; Halimochirurgus, 

 Ale. — Indo-Pacific. 



B. The body is somewhat compressed, and covered with spiny scales 

 or scutes. The co-ossified pelvic bones bear only a single median spine 

 suspended in a scaly fin-fold ; but even this spine may be lost (Mona- 

 eanthidae). The premaxilla is firmly fixed to the maxilla. 



^ 



■%M; 





Fig. -147. 

 Triacanthus brevirostris, T. and S. (After Bay, Fishes of India.) 



Family Balistidae. The 'Trigger-Fishes' have two or three dorsal 

 seines, and generally large scales or scutes. 



Acanthoderma, Ag. ; Oligocene, Europe. Balistes, Cuv. (Figs. 448-9) ; 

 Paralutercs, Blkr. ; Alutere*, Cuv. ; Psilocephalus, Sw. — warm seas. 



Family Monacanthidae. The ' File-Fishes ' have one strong dorsal 

 spine, and generally a second vestigial spine behind it. The pelvic fin 

 is vestigial or absent. The body is covered with small spiny scales. 



Monacanthus, Cuv. (Fig. 450). 



Series 2. (Ostracodermi). There is no spinous dorsal fin, and the 

 whole body, excepting the jaws, base of the fins, and caudal pedicle, is 

 encased in a rigid carapace of hexagonal plates. The vertebrae are reduced 

 to 16-14 ; the epipleurals and the pelvic girdle and fins have disappeared. 

 The clavicles, coracoid, and postclavicle are greatly expanded below the 

 carapace. The hyopalatine arch is fixed. The premaxilla is firmly 

 united to the maxilla. 



