1902.] PLECTOGNATHOUS FISUES. 287 



Spinotis dorsal, if present, of few rays ; no anal spines; ventrals, 

 if present, each represented by a spine (rarely with the addition 

 of one or two rudimentary soft I'ays). Caudal rays in small 

 number, ten to eighteen. 



Four families : Triacanthidse, Triodontidse, Balistidse, and 

 Ostraciontidee. 



In the fishes of this division the pectoral arch is very similar 

 to that of the Acanthuridse, except that the post-temporal is 

 completely united suturally to the squamosal. The vertebral 

 column also is like that of Perciform fishes ; and although Dareste 

 has ascribed to Triodon diapophyses on the posterior prtecaudal 

 and most of the caudal vertebrae, it is evident, from studying his 

 figure and comparing with skeletons of Triacanthus and Bcdistes, 

 that he is referring to the praezygapophyses, which are somewhat 

 enlarged in this region in all these fishes. 



Family 1. Triacanthid^. 



Praecaudal vertebrae with parapophyses from the third or fourth 

 to the last ; epipleurals present. Praeorbital not ossified ; ethmoid 

 region high, a large nasal cavity bounded by ethmoid and pras- 

 frontal ; palatine arch firmly united to the skull ; premaxillaries 

 protractile, free from the maxillaries ; teeth in the jaws separate, 

 conical or incisor-like ; palate toothless ; fourth upper pharyngeals 

 toothed ; lower pharyngeals separate ; opercular bones reduced, 

 but with their normal relations. Pelvis present, firmly united to 

 the pectoral arch. Two nostrils on each side. Four gills, a slit 

 behind the fourth ; pseudobranchiae present ; six branchiostegals. 

 Scales small, sometimes spinate or osseous. Spinous dorsal with 

 two to six spines ; soft dorsal and anal of moderate length or 

 rather short ; ventrals each represented by a strong spine, with 

 an inner basal knob which locks it when everted, rarely with the 

 addition of one or two rudimentary soft rays. Air-bladder 

 present. 



Genera. 



1. TuiACA^'THUS '^ Ouv. — Body compressed, caudal peduncle 

 long and slender. Scales smaU, rough. Lateral line present. 

 D. lY-Y, 22-25 ; A. 16-20. Ventrals without soft rays. 

 Caudal forked, with 12 rsijs. A series of incisors in each jaw, 

 with a few inner rounded teeth. Twenty vertebrae. 



2. Triacaxthodes ^ Bleeker. — Body compressed, caudal 

 peduncle short. Scales small, juxtaposed. No distinct lateral 



1 The Oligroceiie genus Acantlwpleuriis Agassiz has a rounded caudal, but in 

 other respects seems scarcely different from Triacanthus. 



" Spitiacantlms Agassiz, from the Eocene of Monte Bolca, may belong to this 

 family. It resembles Triacanthodes in its dorsal, anal, and caudal fins, except that 

 the six dorsal spines are very long and strong. The eye is placed high, below the 

 first dorsal spine, and the teeth are stout and conical. The pelvis and ventral fins 

 were apparently not strongly developed, and Gill considers this fish to be the type of 

 a separate family. 



