98 VERTEBRATE SKELETON 



branchial region being occasionally absent {Lophius), the arches 

 ending freely below. 



Another Teleostome characteristic is the presence of an opercular 

 apparatus supported by membrane bones connected with the 

 posterior side of the hyoid arch and covering the external openings 

 of the gill-clefts. In its fullest development (figs. 74, 117) it consists 

 of two parts, an upper operculum (gill cover) proper and a lower 

 branchiostegal portion. The most constant of the bones of the upper 

 part is an operculare, (really a lateral line bone, p. 74), articulated 

 to a process of the hyomandibula (fig. 103). The operculare is 

 usually overlaid in front by a preoperculum which also covers the 

 outer side of hyomandibula and quadrate. Below and in part 



Fig. 104. — Hyale and branchiostegals of cod. b, basihyoid; br, branchiostegal rays; 

 c, ceratohyoid; e, epihyoid; h, hypohyal; u, urohyal. 



medial to the operculare is the suboperculum, and ventral to all of 

 these and in the angle between them is the interoperculum. Only 

 the operculare occurs in Chondrostei. 



The branchiostegal part of the apparatus consists of a branchio- 

 stegal membrane supported by a series of usually slender membrane 

 bones, the branchiostegal rays, attached largely or wholly to the 

 ceratohyal. With these last are to be associated the gular (jugular) 

 plates of many recent and fossil Ganoids and Elopidae. In the 

 extinct genera there may be a series of these along the inner margin 

 of each half of the lower jaw, as far forwards as the symphysis. The 

 number is reduced in modern Holostei, there being but a pair or 

 (Amia) a single median plate. 



GANOIDEA.— The skull of the lower Ganoids (Chondrostei) is 

 very like that of Elasmobranchs, especially in the suspension of the 

 jaws, ventral position of the mouth, the pterygoquadrate upper jaw, 

 and the slight development of membrane bones. There are few 

 cranial features which mark the group as a whole from the lower 

 Teleosts, among them the presence of gular plates. 



