FISHES OF THE FAMILY SIGANID^. 2\J 



Xesurus) 1 the parapophyses developed on all the vertebrae ; the 

 postclavical composed of a single piece, though of a similar long, 

 slender shape to that of Siganns. 



In spite of these differences Siganus appears to be more closely 

 related to the family Acanthuridae than to any other known 

 group. It resembles the fishes of the family Acanthuridae in 

 having the maxillary and mandibular elements short and resem- 

 bling each other in shape ; the maximillary and premaxillary 

 solidly attached to each other ; the spines from the symphysis 

 of the latter short ; and the movement of the maxillaries similar 

 to that of the mandible ; the teeth flattened, in a single row, and 

 without entire edges ; the suborbitals without a shelf below the 

 eye-ball ; the gill-filaments extending free above the gill arches 

 in a cavity at each side of the cranium ; a spine developed for- 

 ward from the first interneural spine ; the gill openings restricted 

 to the sides. 



In the connection of the maxillary elements to the cranium 

 Siganus resembles Balistes and the other plectognathous fishes, 

 in which the maxillary elements fit against the concave front of 

 the ethmoid region and have a hinge-like movement without any 

 sliding motion forward. In the acanthuroid fishes the ethmoid 

 region is hemispherical in front, and the maxillary elements fit 

 socket-like over it, with very short premaxillary spines above. 

 A movement similar to that in Siganus is produced by the max- 

 illary elements turning on the ethmoid ball so that the premax- 

 illary spines glide over the ethmoid somewhat as in the majority 

 of fishes, but there is no sliding motion straight forward. 



In most characters, however, the acanthuroid fishes resemble 



Balistes more closely than does Siganus. The shape of the 



cranium is strikingly similar: tapering forward; the parasphenoid 



extending down in a wide thin plate ; the postorbital region 



shortened ; the sphenotic and prefrontal regions curving around 



the eye ; and the postclavicle a long simple ray of bone. 



In all of the plectognathous fishes the premaxillary is firmly 



1 The attachment of the post-temporal to the cranium apparently has not the im- 

 portance sometimes given to it as Dr. Gill has shown in his paper on the affinities of 

 the Ephippiids (Proc. U. S. Nat. Mus., Vol. V., p. 557), or as the present writer has 

 shown in his work on the shoulder girdle of the Hemabranchiate fishes (Proc. U. 

 S. Nat. Mus., Vol. XXV., p. 619). 



