226 THE DOG-FISH. 



1. The first or mandibular arch is very greatly modified ; 



its ventral part is large and stout and forms the 

 lower jaw. The upper jaw is formed as an out- 

 growth from its anterior edge. 



a. The upper jaw consists of a pair of stout cartilaginous 



rods, wide behind and narrowing towards their 

 anterior ends, where. they are united across the 

 middle line by ligament. They extend forwards 

 as far as the openings of the olfactory capsules, 

 and are connected by stout ethmo-palatine liga- 

 ments with the base of the skull, at the antero- 

 inferior angles of the orbits. They bear teeth 

 along the ventral surface of the anterior two- 

 thirds of their length, but these are lost in skulls 

 prepared as described. 



b. The lower jaw consists of a pair of wide flattened 



bars of cartilage, which are movably hinged with 

 the upper jaw behind, and bound together by a 

 median ligament in front. It lies, when the 

 mouth is shut, parallel to and immediately 

 behind the upper jaw. It bears teeth along the 

 dorsal surface of the anterior two-thirds of its 

 length on each side, but these are lost in skulls 

 prepared as described. 



2. The second or hyoidean arch is incomplete above : it 



consists of a pair of rods the dorsal ends of which 

 articulate with the sides of the skull, while the ventral 

 ends are connected together by > median plate of 

 cartilage. Each rod is further divided into an upper 

 and a lower segment. 



a. The hyo-mandibular cartilage, or upper segment, is 

 a short stout bar of cartilage, projecting almost 

 horizontally outwards from the skull, but slightly 

 downwards and backwards. Its inner and upper 

 end articulates with a concave surface on the side 

 of the skull, near its hinder end, and immediately 



