BONES FORMING OPERCULAR SERIES OF FISHES 63 



mandible. This plate, bearing as much resemblance to the 

 opercula as to the branchiostegals, may be named the branchio- 

 perculum; its fold, the branchiopercular fold. The remaining 

 ten plates of the series are attached to the ceratohyal element of 

 the hyoid arch, near the lower edge of its outer face. Except for 

 its pointed, rather than truncate anterior edge, the uppermost of 

 the ten is similar in form to the branchioperculum, which overhes 

 its dorsal edge. Both this plate and the next, which is similar, 

 though only about half as wide, are imbricate on the one next 

 below, having free folds along their lower edges. The lower 

 anterior seven branchiostegals form a continuous even surface, 

 each, except the uppermost, fitting tightly into a groove along 

 the lower outer face of the one next above ; they increase in width 

 anteroventrally. The most anterior branchiostegal, which is 

 wider on the left or outer than on the right side, may be homolo- 

 gous with the branchigular plate of the Palaeoniscidae, or may 

 represent two or three fused branchiostegals. The current alloca- 

 tion of Amia in a position intermediate between the typical 

 ganoids and the teleosts is confirmed by the study of its opercular 

 and branchiostegal plates. 



The Isospondyli, comprising the oldest and most primitive^ of 

 the teleosts, retain certain generalized features of the opercular 

 series. Thus, in Elops an intergular plate is developed, and in 

 Albula, although the plate itself is lacking, the intergular fold 

 remains. The branchiostegals of the typical Isospondyli (at 

 least the upper ones), persist as thin wide plates. The uppermost 

 and widest ray (which may be termed the branchioperculum, as 

 it seems to be homologous with the plate in Amia to which that 

 name is here applied) is attached closely to the inner margin of 

 the sub- and interoperculum ; not having become concealed under 

 these bones, it remains visible from the side. The whole series, 

 in fact, remaining scarcely at all folded together after the fashion 

 of a fan, is visible from below,^ though the branchial membranes 



• 1 Excepting of course Lepidosteus and Amia, if these be included in the 

 Teleostei. 



^ In the clupeoid fishes the expanded preoperculum covers the larger portion 

 of the middle rays, and all of the ra^^s are mostly concealed in Chirocentrus 

 dorab. 



JOURNAL OF -MORPHOLOGY, VOL. 33, NO. 1 



