62 CARL L. HUBBS 



branchiae, is retained in certain extinct groups of ganoids, such 

 as the Palaeoniscidae. Their structure in this family has been 

 described by Traquair and others. On each side is a curved series 

 of homologous, suturally conjoined plates, of which the upper 

 two or three are dilated to form the opercular bones, while the 

 lower anterior ones, narrower and less modified, comprise the 

 branchiostegal rays (or branchiostegals) ; the lowest or most 

 anterior pair, enlarged to form the plates which may be termed 

 the branchigulars (or branchigular plates), are suturally united 

 posteriorly with one another, and anteriorly with the median 

 gular plate (or intergular), which extends forward between the 

 mandibular rami to their symphysis. 



In Amia calva the bones of the opercular series are modified 

 in several respects. Each of the upper five plates is imbricate 

 over the one next below. The gular plate largely covers the 

 most anterior branchiostegal rays of the left side, which in turn 

 overlap those of the right side — a feature due to the definite 

 asymmetric folding of the branchial membranes, an asymmetry 

 which is more or less definitely retained throughout the entire 

 teleost series. The uppermost and largest bone of the opercular 

 series in Amia is the operculum, a large subquadrate plate, in- 

 curved and thickened along its dorsal and anterior edges, and 

 provided inside the anterior edge with an oval socket, into which 

 a peg-like condyle of the hyomandibular fits, allowing consider- 

 able lateral movement of the operculum. The suboperculum, the 

 next lower element of the series, is broadly incised by a downward 

 extending arm of the operculum; its dorsal edge slips just under 

 the lower margin of the operculum, while its lower edge is closely 

 joined by membrane with the triangular interoperculum. The 

 anterior edges of these three plates fit into a posterior groove of 

 the preoperculum, which is not regarded as a member of the 

 opercular series. A free dermal fold connecting the opercular 

 membrane with the mandible, and extending along the lower 

 edge of the sub- and interoperculum, may be termed the sub- 

 opercular fold. The fourth bone of the series, fitting between 

 the sub- and interoperculum, but widely exposed, being nearly 

 one-third as wide as long, extends downward and forward, lying 

 free in a conspicuous fold, continuous with the lower edge of the 



