skeleton: i6i 



In the teleostomous fishes an operculum or fold covering the 

 gill slits occurs ; and this is supported by opercular bones, which 

 in their full development may number four on either side, — oper- 

 culum, preoperculum, interoperculum, and suboperculum. These 

 are cartilaginous in origin, and are usually regarded as extremely 

 modified branchiostegals of the hyomandibular. 



The membrane bones which complete the lateral walls and 

 roof in the cranium are : the dorsal part of the supraoccipital 

 (when distinct called paroccipital), and, proceeding forwards, 

 a pair each of parietals, frontals, and nasals, meeting in the 



Fig. 170. Hyoid and opercular apparatus of cod. B, branchiostegals; E, os 

 entoglossum; H, hyoid; HM^ hyomandibular; /, interoperculum; O, operculum; 

 P, preoperculum ; S, suboperculum. 



middle line above, the skull being terminated by a pair of pre- 

 maxillaries, which also appear on the ventral surface. Lodged 

 in the angle between nasal and frontal is a prefrontal on either 

 side, while a pair of postfrontals are placed in a similar position 

 between the frontals and parietals. Pre- and postfrontals may 

 make up the superior or inner margin of the orbit, or a supra- 

 orbital may intervene between them. Below the postfrontal 

 and behind the orbit there may be a postorbital which may 

 extend beneath the orbit, or the posterior margin of the orbit 

 may be formed by a squamosal (temporal), which extends up- 

 wards in front of the otic region to reach the supraoccipital and 

 parietal. A lachrymal bone is more constant than some that 



