THE COD. 99 



common to the first three arches, the hypo-branchials of which 

 articulate with it laterally : the hypo-branchials of the fourth 

 arch are united with one another in the middle ventral 

 line by cartilage. 



62. The rudimentary fifth branchial arches, each consist- 

 ing of a single bone the inferior pharyngeal bone 

 which is beset on its dorsal face with teeth, and in 

 the entire fish, bites against the superior pharyngeal 



( l62 > l6 3)- 



XII. In the skeleton of the gill-cover or operculum, 



note the following four bones on each side : 



63. The opercular (Fig. 29, <?/), a flat bone, bifid 

 posteriorly and having on its anterior border a facet for 

 articulation with the posterior or opercular process of 

 the hyomandibular. 



64. The pre-opercular (p.op\ a large bone with an 

 evenly curved anterior border produced at one place into 



an irregular process, and a jagged posterior border; its outer 

 face is produced into a prominent ridge for attachment of 

 muscles. In the natural position of the parts it fits closely 

 against the outer face of the hyomandibular, symplectic, 

 and quadrate. 



65. The' sub-opercular (s.op), a flat plate, somewhat 

 rounded below and pointed above : and lies immediately 

 beneath the opercular. 



66. The inter-opercular (t.op), an irregular bone, 

 articulated at its upper and posterior end with the sub- 

 opercular, and at its opposite extremity connected by ligament 

 to the angle of the mandible. 



XIII. There still remain a few loosely attached bones 

 in connection with the brain-case, chiefly in the 

 region of the nose and eyes ; these are 



67. The so-called nasals (no), delicate scale-like bones, 



H 2 



