JAWS 349 



rami of the lower jaw, and separated by a narrow ' isthmus ' 

 supported by the basibranchials and urohyal. 



The opisthotic varies in a remarkable manner, being very large 

 in some (Gadidae), small in others (Esocidae, etc.), or absent 

 (Mormyridae, etc.). An interorbital septum is present (p. 324), 

 except in the Cypriniformes (Ostariophysi ; see, however, the 

 Galaxiidae and Gadidae). In what is probably its more primitive 

 condition it is formed partly by cartilage, partly by the orbito- 

 sphenoids meeting and fusing in the middle line (Salmonidae) ; in 

 other cases it is entirely bony (Allula, Fig. 329), or almost entirely 

 membranous (Elopidae, Gadidae, etc.). The basisphenoid, paired 

 or median, is always small and often absent. The orbitosphenoid 



Albula conorhynchuis, B. and S. Left hyopalatiue arch, etc. ; inner view. (From Ridewood, 

 Proc. Zool. Soc.) an + tear, angular fused to dermal articular; </, dentary ; ewr, dermal 

 articular; ecp, ectopterygoid ; cnar, articular (endosteal) ; cup, endopterygoid ; hm, hyomandi- 

 bular ; iop, interopercular ; mpt, metapterygoid ; oj - , opercular ; pi, palatine ; pop, pre- 

 opercular ; q, quadrate ; s.ar, sesamoid articular ; sop, subopercular ; sy, symplectic. 



is rarely found outside the lowest sub-orders. The palatine 

 articulates in front either by a single large head with the ethmoid 

 cartilage or the prefrontal bone, or by two heads as in the 

 Salmonidae and others (Swinnerton [431]); or the anterior 

 articulation alone remains (Gasterosteus, Belone, etc.). Various and 

 peculiar are the modifications undergone by the mouth and jaws in 

 the Teleostei. The premaxillae become very movable, free behind, 

 and loosely articulated in front in the higher forms. The maxilla 

 loses its teeth and ceases to occupy the margin of the mouth. 

 The two bones come to lie parallel to each other ; the former 

 articulating with the anterior mesethmoid region, the latter with 

 the vomer and lateral ethmoid region and palatine (Fig. 456). 

 Frequently the jaws are protrusible (p. 375), in which case the 

 dorsal process of the premaxilla is usually much elongated and 



