THE SKULL OF THE SALMON. 



middle part forms the cerato-hyal (fig. 11, 27), in front of 

 which is the small hypo-hyal. The hyoid arches of the two 

 sides are united by the large tooth-bearing glosso-hyal (fig. 11, 

 29). Attached to the lower surface of the hyoid arch are a series 

 of twelve flat branckiostegal rays (fig. 11, 35). Each overlaps 

 the one in front of it, the posterior one being the largest. The 

 branchiostegal rays of the two sides are united in front by an 

 unpaired membrane bone, the basi-branchiosteyal (fig. 11, 36). 



Opercular bones. Behind the hyomandibular there is a 

 large bony plate, the operculum, formed of four large mem- 

 brane bones. The anterior of these, the pre-opercular (fig. 11, 

 33), is crescentic in shape, and with its upper end a small 

 supra-temporal (fig. 11, 34) is connected. 



Behind the upper part of the pre-opercular is the largest of 

 the opercular bones, the opercular proper. Its lower edge 

 overlaps the sub-opercular, and both opercular and sub- 

 opercular are overlapped by the infra- opercular (fig. 11, 32) 

 in front. The infra -opercular is in its turn overlapped by 

 the pre-opercular. 



BRANCHIAL ARCHES. 



There are five branchial arches, the first four of which 

 bear gill rays. Each of the first three consists of a shorter 

 upper portion directed obliquely backwards and outwards, 

 and a longer lower portion forming a right angle with the 

 upper and directed obliquely forwards and inwards. The 

 greater part of each arch is ossified. 



The upper part of either of the first two consists of a short 

 tapering pharyngo-branchial directed inwards, and of a 

 long epi-branchial tipped with cartilage at both ends. The 

 junction of the upper and lower parts is formed by a carti- 

 laginous hinge-joint between the epi-branchial and cerato- 

 branchial. The cerato-branchial is a long bony rod sepa- 

 rated by a short area of cartilage from the hypo -branchial, 

 which is succeeded by the basi-branchial meeting its fellow 



